feat(reaper): job-reaper + stale merge-lease reclaim + idempotent merge finalization

Closes the "zombie jobs" incident class: job status was set only inside
the live launcher process, so a process death left jobs.status='running'
forever; at max_concurrency=1 one zombie blocked ALL projects' queue
(self-hosting risk). Adds a background daemon (src/job_reaper.py) with
three-tier liveness (dead-pid streak / known exit_code / max-running
backstop) whose only mutating write is an atomic terminal flip guarded by
WHERE status='running' (no double-process). For exit0 the canonical QG is
the source of truth via gate-driven advance, not "exit0".

Also proactively reclaims stale merge-lease (dead pid OR TTL) via file
delete only (no git ops), and makes merge finalization idempotent
(pr_already_merged guard + up-to-date short-circuit on re-drive).

New jobs.pid column via idempotent _ensure_column (no migration); pid
stamped in launcher._spawn after Popen. Reaper start/stop in lifespan;
"reaper" snapshot in GET /queue. Kill-switches: ORCH_REAPER_ENABLED,
ORCH_REAPER_INTERVAL_S, ORCH_REAPER_DEAD_TICKS, ORCH_REAPER_MAX_RUNNING_S,
ORCH_LEASE_RECLAIM_ENABLED.

Invariants unchanged (AC-13): STAGE_TRANSITIONS, QG_CHECKS registry,
check_branch_mergeable signature/behaviour, BUG-8 rollback, hook exit
codes. restart-safe, never-raise per unit of background work.

Docs: docs/architecture/README.md, CHANGELOG.md, .env.example.
Tests: tests/test_job_reaper.py, tests/test_merge_lease_reclaim.py,
tests/test_merge_gate.py (TC-16), tests/test_merge_gate_race.py (TC-17),
tests/test_queue.py, tests/test_config.py (TC-19/TC-20). 742 passed.

Refs: ORCH-065

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/config.py

@@ -296,6 +296,33 @@ class Settings(BaseSettings):
     post_deploy_auto_rollback: bool = False
     post_deploy_base_url: str = "http://localhost:8500"
 
+    # ORCH-065: job-reaper + proactive merge-lease reclaim. A background daemon
+    # thread (modelled on the reconciler) makes "the monitor thread / process died
+    # while a job/lease was held" self-heal WITHOUT a restart. Status (done/queued/
+    # failed) is otherwise only ever set by launcher._monitor_agent -> _finalize_job
+    # inside the live process; a death there left the jobs row 'running' forever and
+    # (at max_concurrency=1) wedged the queue of EVERY project (incidents 07.06: jobs
+    # 236/239/242/254). The same thread proactively reclaims a stale/dead merge-lease
+    # (ORCH-043) instead of waiting for the lazy TTL on the next foreign acquire. See
+    # docs/architecture/adr/adr-0011-job-reaper-lease-reclaim.md.
+    #   reaper_enabled       -> global kill-switch (false -> strictly prior behaviour;
+    #                           only the startup requeue_running_jobs remains).
+    #   reaper_interval_s    -> background scan period (seconds).
+    #   reaper_dead_ticks    -> Tier-1: consecutive ticks a job's pid must be dead
+    #                           before it is reaped (>=2 anti-false-positive; a live
+    #                           long-running agent is NEVER reaped).
+    #   reaper_max_running_s  -> Tier-3 backstop ceiling: a job 'running' longer than
+    #                           this is reaped even when liveness is unknowable. MUST be
+    #                           > max agent_timeout + grace so a legit agent is safe.
+    #   lease_reclaim_enabled -> kill-switch for the proactive stale/dead lease reclaim
+    #                           (false -> only the legacy lazy TTL reclaim in acquire).
+    # (reuse) merge_lock_timeout_s -> lease TTL; merge_gate_repos -> reclaim scope.
+    reaper_enabled: bool = True
+    reaper_interval_s: int = 60
+    reaper_dead_ticks: int = 2
+    reaper_max_running_s: int = 3600
+    lease_reclaim_enabled: bool = True
+
     # Telegram notifications
     telegram_bot_token: str = ""
     telegram_chat_id: str = ""