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>

tests/conftest.py

@@ -94,4 +94,8 @@ def _disable_merge_verify(monkeypatch):
     """
     from src import config as _cfg
     monkeypatch.setattr(_cfg.settings, "merge_verify_enabled", False, raising=False)
+    # ORCH-073: the regression guard (check_main_regression) runs real git in
+    # _handle_merge_verify's confirmed branch. Default it OFF too so unrelated
+    # deploy->done tests stay 1:1; the dedicated ORCH-073 tests re-enable it.
+    monkeypatch.setattr(_cfg.settings, "regression_guard_enabled", False, raising=False)
     yield