feat: hard merge gate by default (CI + directory + contract + coordinator + reviews)

- cloud/merge_gate_hard.py (new): pure hard gate; mode resolution is
  env-only (unset/invalid -> hard, fail-closed); five checks plus
  branch protection must be protected; unknown/unprotected block
  even with explicit confirm; no-evidence blocks (fail-closed);
  directory check reuses #209 check_directory_isolation (v0.3 rule);
  blocks codes + merge_gate_blocked audit detail are the track-3
  pushback interface (no pushback state machine in this track).
- cloud/server.py: POST /tasks/{id}/merge uses hard mode by default;
  record/enforce/off only when explicitly set via env (preserving
  #209 semantics); body merge_gate_mode is ignored server-side so
  callers cannot downgrade the gate; every gate block writes a
  merge_gate_blocked audit before returning 409, status unchanged;
  hard-mode body type errors return 400.
- docs/merge-gate-branch-protection.md (new): protection payload
  (PR + strict status checks + no force-push + no deletion +
  enforce_admins), empty required_checks = unconfigured, Pro/public
  prerequisites, 403 causes; states protection is NOT claimed enabled.
- SKILL.md (gate-mechanism wording only): four checks -> five checks;
  body + coordinator + client appendix gate paragraphs updated to
  default hard, merge_gate_blocked audit, payload doc reference.
- tests: 31 new hard-gate tests (pure + endpoint + skill text);
  #209 endpoint tests now set record env explicitly (their intent);
  test_server_merge 200-path calls now carry full hard evidence.

cloud/tests/test_merge_gate_protectskill.py

@@ -261,6 +261,10 @@ class MergeEndpointGateTestCase(unittest.TestCase):
         return task
 
     def test_no_gate_fields_behaves_like_old_endpoint(self):
+        # 本用例测的是 record 模式的过渡语义（#209 原意）：默认已是
+        # hard，见 test_merge_gate_hard.py 的默认 409 用例；这里显式
+        # 切 record 才沿用“无证据放行、只附带门禁信息”。
+        os.environ["CLOUD_MERGE_GATE_MODE"] = "record"
         task = self._completed_task()
         status, merged = self._req(
             "POST", "/tasks/%s/merge" % task["id"], {}, self.owner_key)
@@ -283,6 +287,8 @@ class MergeEndpointGateTestCase(unittest.TestCase):
         self.assertEqual(got["status"], "completed")
 
     def test_unprotected_confirm_allows_and_audits_risk(self):
+        # record 模式语义：硬模式下未保护 + 确认仍阻断，见硬门禁测试。
+        os.environ["CLOUD_MERGE_GATE_MODE"] = "record"
         task = self._completed_task()
         status, body = self._req(
             "POST", "/tasks/%s/merge" % task["id"],