feat: add session-owned worktree cleanup audit and TTL enforcement (Closes #401)

Stale worktrees accumulate under branches/ when runs stop early, race a
sibling session, hit a validation failure, or lose cwd state, making later
workflow decisions harder and riskier.

Changes:
- worktree_cleanup_audit.py — ownership metadata, safety-first classifier
  (active_open_pr, active_issue_work, dirty_local_worktree,
  clean_stale_removable, detached_review_leftover, unsafe_unknown), TTL
  expiry, per-worktree removal proof, and success-completion cleanup plan.
  Only clean_stale_removable and detached_review_leftover are removable;
  dirty/PR/leased/protected/unknown worktrees are never auto-deleted.
- gitea_audit_worktree_cleanup — read-only MCP tool that classifies every
  branches/ entry, fetches live open-PR heads (fails closed if unavailable),
  and returns counts, removable candidates, and git worktree list proof.
- work-issue.md 22A + review-merge-pr.md 28 — cleanup/TTL workflow rules.
- tests/test_worktree_cleanup_audit.py — 30 cases covering all nine
  acceptance scenarios plus inference, metadata, TTL, and report accuracy.

Validation:
  /Users/jasonwalker/Development/Gitea-Tools/venv/bin/python \
    -m unittest tests.test_worktree_cleanup_audit tests.test_merged_cleanup_reconcile -q
  38 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -849,6 +849,16 @@ Confirm:
 
 Clean only the session-owned `branches/` review worktree if the project workflow explicitly allows cleanup.
 
+Review, baseline, and merge-simulation worktrees created during this run are
+transient and are removed automatically at successful completion once they are
+clean, carry no open PR, and hold no active lease (#401). Use
+`gitea_audit_worktree_cleanup` (read-only) to classify `branches/` entries; only
+`clean_stale_removable` and `detached_review_leftover` may be removed, one-by-one,
+after `git worktree list` proof plus per-worktree proof of path, branch/HEAD,
+clean/dirty status, no active PR/lease, and the removal result. Dirty,
+active-PR, active-issue, and leased worktrees are never deleted automatically; a
+failed removal must be reported with the leftover path and reason.
+
 Do not delete or mutate unrelated branches/worktrees.
 
 Do not touch the main checkout except to update the stable branch after merge if explicitly allowed by the workflow.