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/config.py

@@ -130,6 +130,28 @@ class Settings(BaseSettings):
     ci_poll_max_attempts: int = 12
     ci_poll_interval_s: int = 10
 
+    # ORCH-043: merge-gate (auto-rebase + re-test + merge-lock) on the
+    # deploy-staging -> deploy edge. A deterministic sub-gate (no LLM) that
+    # catches the up-to-date branch up to the CURRENT origin/main, re-tests it,
+    # and serialises merges so two green branches can't break main.
+    #   merge_gate_enabled     -> global kill-switch; False -> no-op pass for the
+    #                             whole gate (staged rollout, env ORCH_MERGE_GATE_ENABLED).
+    #   merge_gate_repos       -> CSV of repos where the gate is REAL; empty means
+    #                             only the self-hosting repo (orchestrator). Other
+    #                             repos -> conditional no-op (mirrors ORCH-35 staging).
+    #   merge_retest_timeout_s -> wall-clock budget for the post-rebase re-test.
+    #   merge_retest_target    -> pytest target for the re-test (portability across repos).
+    #   merge_lock_timeout_s   -> max lease age; an older lease is reclaimed (crash backstop).
+    #   merge_defer_delay_s    -> delay before re-running the gate when the lock is busy.
+    #   merge_defer_max_attempts -> defer retries before escalation (avoids livelock).
+    merge_gate_enabled: bool = True
+    merge_gate_repos: str = ""
+    merge_retest_timeout_s: int = 600
+    merge_retest_target: str = "tests/"
+    merge_lock_timeout_s: int = 300
+    merge_defer_delay_s: int = 60
+    merge_defer_max_attempts: int = 5
+
     # Telegram notifications
     telegram_bot_token: str = ""
     telegram_chat_id: str = ""