Keep a title a person set when the Planner drafts the document heading

[lavaman131]

A member created a document titled "E2E5 2026-10-16: verify login resume". When the Planner later drafted the title and goal in the background, it replaced that title with "Verify login logic in auth.rs", and the slug changed with it.

Cause

The background heading job's draft_heading tool (apps/server/src/agent/heading-tool.ts) wrote the drafted title over whatever H1 the document had. The prompt even invited it to "refine even a human-chosen title". Nothing recorded who set the current title.

Change

• draft_heading now replaces the title only in three cases:
  • the title is blank;
  • the title is still the generated adjective-noun name of a document created without a title;
  • the title equals the one a Planner last drafted.
• The Planner's drafted title is stored in the sidecar as plannerTitle and survives a reopen. Any other title counts as a person's, whether they set it at creation, by renaming, or by typing in the heading. A person's title is never replaced.
• When the title is kept, the draft can still seed a goal into an empty body. The activity notice then reads "Chopin drafted the goal" instead of "Chopin drafted the title and goal".
• When a person chose the title, the heading prompt tells the job to keep it. The tool enforces the rule regardless of what the model does.
• The README and docs/hosted-agent.md describe the new rule.
• Some test fixtures relied on the old behaviour: they named documents "Friendly Lark", "Test …" or "Test plan" and expected a draft to replace that name. They now start with a blank or generated title.

The foreground edit_plan tool is unchanged. A title it writes is not recorded as plannerTitle, so a later background draft keeps it.

Verification

• Red before the fix:
  • In heading-tool.test.ts, the member-title, unchanged-body and rename-after-draft tests failed because the title was replaced.
  • The new job-prompts and planner-job-support tests failed before the prompt and notice changes.
  • With plannerTitle removed from the sidecar, the reopen-then-refine test failed.
  • The new browser test "a heading job keeps the title a person chose" failed against main's heading-tool.ts.
• bun test apps/server apps/web packages: 2 failures out of 5787 tests. One was a stale assertion, now fixed in runtime-naming.test.ts. The other was an atomic-harness timing failure under load that passes 3/3 when run alone.
• bun run types and bun run ci pass.
• Browser tests:
  • document-title-naming: 2/2 pass, including the new test.
  • The document-title, hosted, harness, conversation-initiative and document-entry specs: 55 of 56 pass. The one failure is document-entry "a parent rename while its child closes…". It also fails 3 of 6 times on main, so it is a pre-existing flake.
  • In the conversation-plan scripted config, the heading and runtime specs pass. All 6 tests in conversation-plan-jobs fail identically on main.