Fix (two layers):
1. Root cause — pr_merge and rebase_pr_for_task now take a required project_id and scope the lookup where(pr_number == X AND project_id == Y). Required so no caller can forget — the bug class can't recur. All 4 call sites updated (choreographer cell_pm_complete, the rebase-retry, the superseded close_pull_request now passes project_id, and _verb_runner._do_pr_merge).
2. Crash guard — _finalize_cell_complete None-checks the complete() return and returns a clean invalid_state envelope (with a remediate hint) instead of dereffing None → 500 → respawn loop.
Two PMs completing different subtasks of the same parent could race
on gh API merge calls. GitHub returns 409 to one but local DB write
ordering wasn't guaranteed. Take row-level lock on parent task before
merge; retry once on 409.
Renn F