Pin per-test durable session-state so patch.dict(clear=True) cannot adopt
host ~/.cache review-decision locks, and clear the merge-test ledger in
setUp. Keeps active #332 hard-stop coverage while making TestMergePR
deterministic without manual host cache cleanup.
Aligned with #590 / PR #592 isolation approach.
Durable review-decision locks (#559) can outlive the PR they protect.
When the last terminal mutation references a PR that is already
merged/closed, expose gitea_cleanup_stale_review_decision_lock so a
reviewer can clear the lock with identity/profile gates and a durable
audit trail — without weakening #332 for open PRs or deleting
session-state files by hand.
Also auto-clear same-profile locks after a successful merge of the
approved PR, and document allowed vs forbidden cleanup.
Closes#594