Add a resolver for getting the default branch

.github/copilot-instructions.md

@@ -21,7 +21,7 @@ No Makefile, no code generation, no external linter config. Standard Go toolchai
 
 - `cmd/`: One Cobra command per file. Each exports `<Name>Cmd(cfg *config.Config)` with logic in `run<Name>()`.
 - `internal/git/`: `Ops` interface wrapping git CLI. `MockOps` for tests. Use `ForWorktree(path)` for scoped execution, `CommonDir()` for shared storage, and `GitDir()` for native per-worktree state. Never use production `os.Chdir` or `SetOps` to switch context.
-- `internal/github/`: `ClientOps` interface (18 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`); merges use the async merge API (`/repos/{owner}/{repo}/pulls/{n}/merge-async`) with an explicit `merge_action` (`direct_merge` or `merge_queue`) chosen from the base branch's merge-queue detection. `merge_action` is optional — omitting it (or sending `default`) lets the server auto-route (merge queue if one is configured, else direct merge) — but the CLI sends it explicitly so a wrong detection fails loudly instead of silently merging directly.
+- `internal/github/`: `ClientOps` interface (19 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`); merges use the async merge API (`/repos/{owner}/{repo}/pulls/{n}/merge-async`) with an explicit `merge_action` (`direct_merge` or `merge_queue`) chosen from the base branch's merge-queue detection. `merge_action` is optional — omitting it (or sending `default`) lets the server auto-route (merge queue if one is configured, else direct merge) — but the CLI sends it explicitly so a wrong detection fails loudly instead of silently merging directly.
 - `internal/config/`: `Config` struct passed to all commands. Holds I/O, colors, and test hooks (`SelectFn`, `ConfirmFn`, `InputFn`, `GitHubClientOverride`).
 - `internal/stack/`: Shared catalog (`<common-dir>/gh-stack`, JSON), conservative legacy migration, atomic saves, and short catalog locks.
 - `internal/worktree/`: Origin/owner identities, cleanliness preflight, scoped operations, and touched-ref recovery.

AGENTS.md

@@ -37,7 +37,7 @@ internal/
     gitops.go                # Ops interface, including scoped worktree execution
     mock_ops.go              # MockOps. Each method has a corresponding *Fn field.
   github/                    # github.ClientOps interface + real Client
-    client_interface.go      # ClientOps interface (18 methods)
+    client_interface.go      # ClientOps interface (19 methods)
     mock_client.go           # MockClient. Uses function-pointer fields for testing.
   stack/                     # common-directory catalog, JSON schema, migration, locking
     schema.json              # JSON Schema for the stack file format
@@ -114,7 +114,7 @@ if errors.As(err, &exitErr) { ... }
 
 - **`git.Ops`** (`internal/git/gitops.go`): wraps git CLI calls. Package-level functions (e.g., `git.CurrentBranch()`) delegate to a swappable package-level `ops` variable. `git.ForWorktree(path)` returns an explicitly scoped executor for HEAD/index/working-file operations. Never switch production context with `os.Chdir` or `git.SetOps`; reserve `SetOps` for tests. `GitDir()` remains per-worktree; `CommonDir()` is repository-wide.
 - **Scoped Git errors:** `ForWorktree(path)` returns `(Ops, error)` and no executor for invalid contexts. Each scoped operation rechecks directory identity. `BranchExists`, `HasStagedChanges`, `IsRebaseInProgress`, and `IsCherryPickInProgress` return `(bool, error)`; callers must handle lookup failures before mutating Git or recovery state, not treat them as absence.
-- **`github.ClientOps`** (`internal/github/client_interface.go`): 18 methods for GitHub API (PRs, stacks, merges). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Async stack merges use `RepoMergeConfig` (GraphQL: allowed merge methods + viewer's default), `BaseBranchUsesMergeQueue` (GraphQL: detects a base-branch merge queue to select the explicit `merge_action`), `MergeStackAsync`, and `GetAsyncMergeResult` (`/repos/{owner}/{repo}/pulls/{n}/merge-async`). Injected via `cfg.GitHubClientOverride` in tests.
+- **`github.ClientOps`** (`internal/github/client_interface.go`): 19 methods for GitHub API (PRs, stacks, merges, repository default branch). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Async stack merges use `RepoMergeConfig` (GraphQL: allowed merge methods + viewer's default), `BaseBranchUsesMergeQueue` (GraphQL: detects a base-branch merge queue to select the explicit `merge_action`), `MergeStackAsync`, and `GetAsyncMergeResult` (`/repos/{owner}/{repo}/pulls/{n}/merge-async`). Injected via `cfg.GitHubClientOverride` in tests.
 - **`config.Config`** (`internal/config/config.go`): Central configuration passed to all commands. Holds I/O streams, color functions, and test hook fields (`SelectFn`, `ConfirmFn`, `InputFn`, `RepoOverride`).
 
 ### Stack file

cmd/add_test.go

@@ -761,7 +761,7 @@ func TestAdd_InitializesStackWithExplicitBranch(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return "unstacked", nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn:    func(name string) (bool, error) { return name == "main" && trunkExists, nil },
 		RevParseFn: func(ref string) (string, error) {
@@ -815,7 +815,7 @@ func TestAdd_InitializesStackWithPromptedBranch(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return "main", nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		CreateBranchFn: func(name, base string) error {
 			createdBranch = name
@@ -856,7 +856,7 @@ func TestAdd_InitializesGeneratedBranchAndCommits(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return currentBranch, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn:    func(name string) (bool, error) { return name == "main", nil },
 		CreateBranchFn: func(name, base string) error {

cmd/init.go

@@ -87,7 +87,7 @@ func runInit(cfg *config.Config, opts *initOptions) error {
 	}
 
 	if trunk == "" {
-		trunk, err = git.DefaultBranch()
+		trunk, err = resolveDefaultBranch(cfg, "")
 		if err != nil {
 			cfg.Errorf("unable to determine default branch\nUse -b to specify the trunk branch")
 			return ErrNotInStack

cmd/init_test.go

@@ -102,7 +102,7 @@ func TestInit_CreatesStackWithCorrectTrunk(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 	})
 	defer restore()
@@ -151,7 +151,7 @@ func TestInit_RestoresMissingLocalTrunkWhenTagResolves(t *testing.T) {
 
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		CurrentBranchFn:   func() (string, error) { return "renamed-branch", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn: func(name string) (bool, error) {
@@ -205,7 +205,7 @@ func TestInit_AdoptExistingBranches(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -231,7 +231,7 @@ func TestInit_ExplicitBranchNamesUsedVerbatim(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -257,7 +257,7 @@ func TestInit_AdoptFlagShowsDeprecationWarning(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -276,7 +276,7 @@ func TestInit_RerereAlreadyEnabled(t *testing.T) {
 	enableRerereCalled := false
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		CurrentBranchFn:   func() (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		EnableRerereFn: func() error {
@@ -308,7 +308,7 @@ func TestInit_RefuseIfBranchAlreadyInStack(t *testing.T) {
 
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "feature-1", nil },
 	})
 	defer restore()
@@ -326,7 +326,7 @@ func TestInit_AdoptNonexistentBranch_CreatesIt(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return false, nil },
 		CreateBranchFn: func(name, base string) error {
@@ -350,7 +350,7 @@ func TestInit_MultipleBranches_CreatesAll(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -375,7 +375,7 @@ func TestInit_AdoptWithExistingOpenPR(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -425,7 +425,7 @@ func TestInit_AdoptIgnoresClosedAndMergedPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -466,7 +466,7 @@ func TestInit_ImplicitAdopt_AllExist(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -501,7 +501,7 @@ func TestInit_ImplicitAdopt_AllMissing(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -528,7 +528,7 @@ func TestInit_ImplicitAdopt_Mixed(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return existing[name], nil },
 		CreateBranchFn: func(name, base string) error {
@@ -556,7 +556,7 @@ func TestInit_WhatsNext_Fresh(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn:  func(name, base string) error { return nil },
 	})
@@ -581,7 +581,7 @@ func TestInit_WhatsNext_AdoptedWithPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -614,7 +614,7 @@ func TestInit_WhatsNext_AdoptedNoPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -639,7 +639,7 @@ func TestInit_WhatsNext_MixedWithPR(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return name == "existing", nil },
 		CreateBranchFn:  func(name, base string) error { return nil },
@@ -670,7 +670,7 @@ func TestInit_Interactive_OnTrunk(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 	})
 	defer restore()
@@ -689,7 +689,7 @@ func TestInit_Interactive_OnFeatureBranch_UseCurrent(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "feat/auth", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return name == "feat/auth", nil },
 	})
@@ -732,7 +732,7 @@ func TestInit_TwoPassValidation_NoBranchCreatedOnError(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -756,7 +756,7 @@ func TestInit_TwoPassValidation_InvalidRefName(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		ValidateRefNameFn: func(name string) error {
 			if name == "invalid..name" {

cmd/link.go

@@ -235,7 +235,7 @@ func runLinkCreateOrUpdate(cfg *config.Config, client github.ClientOps, opts *li
 	// given, default to the repository's default branch (like `gh stack init`).
 	bottomBase := opts.base
 	if bottomBase == "" {
-		base, err := git.DefaultBranch()
+		base, err := resolveDefaultBranch(cfg, opts.remote)
 		if err != nil {
 			cfg.Errorf("unable to determine default branch\nUse --base to specify the base branch")
 			return ErrSilent