Source control: settle threads and clean up worktrees when linked PRs merge

[Tryanks]

Summary

Use linked PR state for settling and worktree cleanup, once Settled and forge support exist.

Spec

• Settle on merge / close (setting auto_settle_on_merge, default on): a thread whose latest-terminal linked PR was closed, or merged with the setting on, is auto-settled — only when the close/merge happened at or after the last user activity (so reopened work is not re-settled). Any visible linked PR that is open or not yet synced blocks auto-settle entirely, inactivity included. A merely open branch PR does not block inactivity settling. Before settling on a terminal PR, re-check the branch for a newly opened PR.
• Worktree cleanup on merge (opt-in): remove a Tcode-managed worktree only when both the host reports the PR merged and git merge-base --is-ancestor HEAD origin/<default> succeeds after a fetch; refuse if the worktree is dirty, shared, has a live terminal, or is not Tcode-managed. Squash merges fail the ancestry check by design; the snapshot/keep-count lifecycle from Worktrees: host-owned ownership and an archive lifecycle (snapshot, transfer, keep-count) #530 still applies.
• The settlement sweep reacts immediately to PR merge/close events.

Dependencies

#526, #532, #530.

Reference

T3 Code ThreadSettlementPolicy.ts (PR rules), #9528, #12161; apps/server/src/storageCleanup.ts (#11598).