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>

.env.example

@@ -17,3 +17,22 @@ ORCH_DB_PATH=/app/data/orchestrator.db
 # one is sent silently to the BOTTOM of the chat (deleteMessage + sendMessage +
 # repoint). One card per task in both modes. Any value other than "bump" -> edit.
 ORCH_TRACKER_MODE=edit
+# ORCH-043: merge-gate (auto-rebase onto current origin/main + re-test + merge-lock)
+# on the deploy-staging -> deploy edge. Deterministic sub-gate (no LLM) that catches
+# the branch up to the CURRENT origin/main, re-tests it, and serialises merges so two
+# green parallel branches can't break main.
+#   ENABLED   -> global kill-switch (false -> whole gate is a no-op pass).
+#   REPOS     -> CSV of repos where the gate is REAL; empty -> only the self-hosting
+#                repo (orchestrator); other repos -> conditional no-op (mirrors ORCH-35).
+#   RETEST_TIMEOUT_S -> wall-clock budget for the post-rebase re-test.
+#   RETEST_TARGET    -> pytest target for the re-test.
+#   LOCK_TIMEOUT_S   -> max merge-lease age before a stale lease is reclaimed.
+#   DEFER_DELAY_S    -> delay before re-running the gate when the lock is busy.
+#   DEFER_MAX_ATTEMPTS -> defer retries before escalation (avoids livelock).
+ORCH_MERGE_GATE_ENABLED=true
+ORCH_MERGE_GATE_REPOS=
+ORCH_MERGE_RETEST_TIMEOUT_S=600
+ORCH_MERGE_RETEST_TARGET=tests/
+ORCH_MERGE_LOCK_TIMEOUT_S=300
+ORCH_MERGE_DEFER_DELAY_S=60
+ORCH_MERGE_DEFER_MAX_ATTEMPTS=5