feat(merge-gate): auto-rebase onto current main + re-test + serialise merges

Deterministic (no-LLM) sub-gate on the deploy-staging -> deploy edge that
catches a feature branch up to the CURRENT origin/main, re-tests the combined
tree, and serialises merges with a per-repo file lease — so two green parallel
branches can no longer break main (self-hosting safety for the orchestrator repo).

- src/merge_gate.py: branch_is_behind_main, auto_rebase_onto_main (push
  --force-with-lease ONLY the task branch, NEVER main), retest_branch, and a
  file merge-lease (atomic O_CREAT|O_EXCL, holder-aware release, stale reclaim).
  Strict never-raise contract; all git ops in the per-branch worktree.
- src/qg/checks.py: check_branch_mergeable composes the primitives under the
  lease; registered in QG_CHECKS. Conditional rollout (merge_gate_enabled /
  merge_gate_repos, default self-hosting only).
- src/stage_engine.py: sub-gate hook on deploy-staging (not a new stage). PASS ->
  advance; "merge-lock busy" -> DEFER (re-queue with available_at, anti-deadlock
  at max_concurrency=1, capped); conflict/red re-test -> rollback to development
  + developer retry (capped by MAX_DEVELOPER_RETRIES). Lease released on
  deploy->done / rollback / PR-merged webhook.
- src/db.py: enqueue_job(available_at_delay_s=...) for the defer (no schema change).
- src/webhooks/gitea.py: holder-aware lease release on PR-merged.
- src/config.py + .env.example: ORCH_MERGE_* settings.

Docs: README + adr-0006 (architect) already cover the design; CHANGELOG updated.
Tests: test_merge_gate.py, test_qg_merge_gate.py, test_merge_gate_race.py,
test_stage_engine.py::TestMergeGate, test_config.py, QG-registry snapshot.
Full suite: 535 passed.

Refs: ORCH-043

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

src/webhooks/gitea.py

@@ -334,6 +334,15 @@ async def handle_pr(payload: dict):
                 logger.error(f"Task {task_id}: max retries reached, needs manual intervention")
 
     elif action == "closed" and pr.get("merged", False):
+        # ORCH-043: the branch's PR just merged into main -> release the per-repo
+        # merge lease this task held from the merge-gate (holder-aware by branch, so
+        # it can't clobber a lease another task acquired afterwards). Never raises.
+        try:
+            from ..merge_gate import release_merge_lease
+            release_merge_lease(repo_name, head_branch)
+        except Exception as e:  # noqa: BLE001 - defensive, never block the webhook
+            logger.warning(f"Task {task_id}: merge-lease release on PR-merge failed: {e}")
+
         # BUG 8 (second door): at the deploy stage `done` is gated by the
         # deployer's verdict (check_deploy_status via advance_stage), NOT by the
         # fact that the PR was merged. The deployer merges the PR at the START of