fix(merge-gate): SHA-in-main as sole merge-verify criterion + main regression guard

Root-cause fix for main erosion (phantom merge): code of ORCH-067/069 reached
`done` while absent from origin/main (only their auto docs-PRs landed).

- FR-1: verify_merged_to_main confirms merge ONLY by `git merge-base
  --is-ancestor <validated_sha> origin/main`; the OR-branch pr_already_merged is
  removed (a merged PR no longer confirms). Empty SHA / git error -> False.
- FR-2: pr_already_merged demoted to merge_pr idempotency-guard; counts a PR only
  when merged & head.ref==<branch> & base.ref=="main" (explicit in-loop filter).
- FR-3: merge_pr selects the open code-PR by head==<branch> AND base==main.
- FR-5: new deterministic check_main_regression in _handle_merge_verify (after
  confirmed SHA-in-main, before done) verifies MAIN_REGRESSION_MARKERS still in
  origin/main; deterministic count==0 -> alert "main regressed" + HOLD (NOT done,
  no rollback); git error of the grep -> fail-open. Kill-switch
  ORCH_REGRESSION_GUARD_ENABLED; non-self -> no-op.
- FR-4: root .gitattributes `CHANGELOG.md merge=union` so Unreleased edits
  auto-merge on rebase without conflict (branch not rolled back).

Invariants unchanged (STAGE_TRANSITIONS, QG_CHECKS, deploy-status, merge-gate,
image-freshness, DB schema, external HTTP API); non-self repos no-op (INV-5);
never-raise (INV-1); merge only via Gitea PR-API (INV-2).

Docs: CHANGELOG, .env.example (README/ADR updated by architect). Tests:
tests/test_orch073_*.py (TC-01..18); existing merge-gate tests updated for the
new code-PR filter.

Refs: ORCH-073

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

.env.example

@@ -50,6 +50,27 @@ ORCH_MERGE_RETEST_TARGET=tests/
 ORCH_MERGE_LOCK_TIMEOUT_S=300
 ORCH_MERGE_DEFER_DELAY_S=60
 ORCH_MERGE_DEFER_MAX_ATTEMPTS=5
+# ORCH-071/073: merge-verify under-gate on the `deploy -> done` edge (врезка in
+# advance_stage, NOT a new STAGE_TRANSITIONS edge / registered QG). A deterministic
+# merge-actor merges the feature code-PR via the Gitea PR-merge API (never push/
+# force-push to main), then `done` is allowed ONLY when the deployed SHA is proven an
+# ancestor of origin/main (ORCH-073 FR-1: SHA-in-main is the single criterion; a
+# merged PR alone no longer confirms). A secondary regression guard then checks a
+# declarative marker set (MAIN_REGRESSION_MARKERS) is still in origin/main; a missing
+# marker -> alert + HOLD (NOT done), a git error of the grep itself -> fail-open.
+#   MERGE_VERIFY_ENABLED      -> global kill-switch (false -> strictly pre-ORCH-071).
+#   MERGE_VERIFY_REPOS        -> CSV of repos where the under-gate is REAL; empty ->
+#                                only the self-hosting repo (orchestrator); non-self -> no-op.
+#   MERGE_PR_TIMEOUT_S        -> per Gitea list/merge HTTP call timeout.
+#   MERGE_VERIFY_TIMEOUT_S    -> git fetch/merge-base timeout for the ancestor + marker checks.
+#   REGRESSION_GUARD_ENABLED  -> kill-switch for the ORCH-073 main-integrity regression
+#                                guard (false -> SHA-in-main alone gates done); reuses the
+#                                merge-verify scope, so non-self repos are a no-op.
+ORCH_MERGE_VERIFY_ENABLED=true
+ORCH_MERGE_VERIFY_REPOS=
+ORCH_MERGE_PR_TIMEOUT_S=60
+ORCH_MERGE_VERIFY_TIMEOUT_S=60
+ORCH_REGRESSION_GUARD_ENABLED=true
 # ORCH-036: executable self-deploy of the `deploy` stage. For the self-hosting repo
 # (orchestrator) the stage REALLY restarts prod (8500) via a detached host hook;
 # deploy_status: SUCCESS means proven health-ok, not an LLM declaration. Three