feat(server): send a message to a thread when it settles

[bogorman]

Problem

Settling means "this thread is done", but the agent has no chance to wrap up: summarise what it did, note what's left,
or report it somewhere. The only settle hook today is the project settle script (pingdotgg#16290). That's a shell command, it runs
after the thread is parked, and it skips threads in the shared checkout.

Change

A new project-scoped setting, Message on settle (settlePrompt, empty by default), under Thread behavior.
When a thread settles (by hand, after inactivity, or on merge), the thread gets that message, for example a skill such as
/wrap-up, as one more turn. When the turn ends, the thread settles again, and settle cleanup (idle shells, the settle
script) runs after that.

• Empty: settling is unchanged.
• Once per settlement: a re-emitted thread.settled with the same settledAt sends nothing more.
• Timeout: a turn still running after 10 minutes is interrupted, then the thread settles.
• Waiting on the user: if the turn leaves the thread blocked (a question, an approval, a queued message), the
  follow-up settle is refused and the thread stays active for them.
• Skipped: archived threads and threads that take no messages (dispatch fails, it's logged, and the thread settles
  as before).
• Where it lives: in ThreadSettlementServiceV2, keyed off the thread.settled and run.* events, so manual,
  automatic and agent-initiated settles all go through it.
• Settings and docs: the field is searchable and project-overridable; the docs are in thread-sidebar.md.

While the wrap-up turn runs, the thread is briefly active (sending a message re-engages it), then it settles again.
Mobile settings don't show the field yet. The server honours it either way.

Testing

• ThreadSettlementService.test.ts has 3 new tests:
  • the prompt is sent, then the thread settles again when its turn ends, then cleanup runs
  • the prompt is sent once when a settlement is re-emitted
  • a turn past the timeout is interrupted, then the thread settles
• Server: 5,498 tests pass and 8 fail. The same 8 fail on unchanged main on this machine (file-system and sandbox
  tests). Contracts: 644 pass. Web: 6,599 pass. Type checks and format are clean.
• Manual check in the desktop app is still to do.

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