fix #475: separate Windows cmd and PowerShell actions

[ahmad-ajmal]

What

Add a Windows-only run_powershell action that accepts raw PowerShell source through script. Windows run_shell now explicitly runs cmd.exe command lines and directs PowerShell callers to the dedicated action. Both tools are included in the Windows core action set.

Reject legacy Windows run_shell requests selecting PowerShell, pwsh, or another unsupported interpreter before launching a process. Existing cmd, default, and auto selections continue to run with cmd.exe.

Allow the validation agent to call run_powershell when enabled. Its definition remains disabled by default. Track the new action in agent-app construction activity, prevent action lookup from resolving implementations for another OS, and distinguish a registered process stopped by the user from a command failure.

Closes #475. Replaces #476 after renaming the source branch to ahmad/fix-windows-shell-guidance.

Why

Wrapping PowerShell source in another interpreter can expand variables or literal percent expressions before execution, or return exit code 0 while printing the intended script as a string. A separate tool makes the interpreter explicit before the agent writes the script and supplies a direct disk-query example.

Native stdout, stderr, and exit codes pass through. Commands are not rewritten or retried automatically, and stderr keywords are not used to infer failure.

How to test

• python -m unittest discover -s tests/windows_shell -v — 21 tests pass locally on Windows. Exercises raw-script quoting, variables, pipelines, multiline input, calculated properties, disk and largest-file queries, literal percent expressions, native errors, stderr and exit codes, cmd compatibility, rejected shell selections, background completion, timeouts, and simulated user stops.
• A focused dispatch check loads the production validator definition and runner dispatch method: the old action list blocks run_powershell; the corrected list executes $n=2; Write-Output $n with stdout 2, and cmd execution still works. Windows-only action lookup remains enforced on Linux and macOS.
• ruff format --check . and ruff check . with Ruff 0.16.1 — pass repository-wide.
• python -m compileall -q app agent_core agents decorators skills — passes.
• git diff --check — passes.

The Windows CI workflow runs the standard-library regression suite with real OS processes, stubbing Node PATH preparation and cancellation registration. Additional pytest registration/cancellation tests are in tests/test_shell_actions.py; the local follow-up environment did not have pytest installed. Full UI-triggered cancellation and end-to-end LLM retry counts are not covered.

Scope

run_powershell uses Windows PowerShell (powershell.exe), with raw source passed directly through -Command. It does not introduce EncodedCommand, temporary scripts, pwsh selection, or advisory result fields. Explicitly launching an interpreter from a cmd command line remains possible and retains that interpreter's normal quoting semantics. Linux and macOS run_shell schemas are unchanged.