Ports #8600's server-owned settlement to orchestration v2 instead of
keeping client-side evaluation. A ThreadSettlementService sweep runs at
startup, on auto-settle settings changes, and once per minute: it
evaluates inactivity and merged or closed pull requests over v2 thread
shells and dispatches the new guarded thread.auto-settle command, which
rejects threads that changed after the sweep's snapshot or carry any
explicit override, then reuses the orchestrator's settle lifecycle.
With the server deciding, the clients drop their effectiveSettled
evaluation and partition on the persisted settledOverride like main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>