code-to10 v6 A1: delete server merge-gate modes, hard only

Server POST /tasks/{id}/merge no longer has record/enforce/off:
hard_mode_from_env and HARD_MODES deleted (not re-defaulted),
handler always evaluates the hard gate, env CLOUD_MERGE_GATE_MODE
and body merge_gate_mode are ignored server-side. The record
'no-evidence passes' and 'unprotected+confirm passes with
merge_gate_risk' server paths are gone with the dead risk audit;
no production caller justified keeping them (only production
caller of the non-hard evaluator was the merge handler itself;
audit_loop_wiring already used the hard gate). cloud/merge_gate
pure functions keep their #209 semantics but are documented as
non-server: they are no longer wired to any endpoint.
Server-level record/enforce/off tests rewritten to the new
behavior (renamed, not deleted silently); pure-function tests
unchanged.

(cherry picked from commit 08e75edbd6d04701965c5db951fa054cfbb75bb0)

Reconcile-port: handler_lifecycle.py hunks redirected into cloud/server.py _merge_task (target keeps 6-mixin split; mixin file does not exist here); handler_support.py kept at HEAD (its A1 hunk is a docstring of _validate_hard_gate_body, ported to server.py copy); test mock paths cloud.handler_lifecycle.* rewritten to cloud.server.*.

cloud/tests/test_v2cov_server_handlers2.py

@@ -546,13 +546,25 @@ class MergeLoopTestCase(_Base):
         self.assertIn("files_changed must be a list", payload["error"])
 
     def test_merge_pushback_unavailable_409(self):
+        # A1 改写：旧用例以 CLOUD_MERGE_GATE_MODE=off + 空 body 走到
+        # 推回检查；服务端删模式后空 body 在硬门禁体校验前已 400，
+        # 故改用完整硬证据 body，仍钉“推回状态不可用 → 409”这一性质。
         self._project()
         self._task(status="completed")
-        h = self._h(body={})
-        with mock.patch.dict(os.environ,
-                             {"CLOUD_MERGE_GATE_MODE": "off"}), \
-                mock.patch("cloud.server._pushback_block",
-                           side_effect=RuntimeError("chain down")):
+        h = self._h(body={
+            "files_changed": ["apps/web/a.py"],
+            "owned_dir": "apps/web",
+            "branch_protection": "protected",
+            "ci_status": "success",
+            "ci_checks": [{"name": "ci", "status": "success"}],
+            "required_checks": ["ci"],
+            "contract_version": "v2",
+            "contract_frozen": True,
+            "coordinator_approved": True,
+            "unresolved_review_comments": 0,
+        })
+        with mock.patch("cloud.server._pushback_block",
+                        side_effect=RuntimeError("chain down")):
             h._merge_task("u1", "t1")
         code, payload = self._last(h)
         self.assertEqual(code, 409)
@@ -561,11 +573,24 @@ class MergeLoopTestCase(_Base):
         self.assertTrue(payload["audit_pushback"]["merge_blocked"])
 
     def test_merge_pushback_unavailable_audit_fails_too(self):
+        # A1 改写：同上，off + 空 body 改为完整硬证据 body，仍钉
+        # “审计本身也失败时 handler 仍回同一 409”。
         # The audit best-effort except-pass: even when the audit
         # write itself raises, the handler still answers the same 409.
         self._project()
         self._task(status="completed")
-        h = self._h(body={})
+        h = self._h(body={
+            "files_changed": ["apps/web/a.py"],
+            "owned_dir": "apps/web",
+            "branch_protection": "protected",
+            "ci_status": "success",
+            "ci_checks": [{"name": "ci", "status": "success"}],
+            "required_checks": ["ci"],
+            "contract_version": "v2",
+            "contract_frozen": True,
+            "coordinator_approved": True,
+            "unresolved_review_comments": 0,
+        })
         real_auth = server_mod._auth_store
         calls = {"n": 0}
 
@@ -574,10 +599,8 @@ class MergeLoopTestCase(_Base):
             if calls["n"] == 1:
                 return real_auth(db_path)
             raise RuntimeError("audit down")
-        with mock.patch.dict(os.environ,
-                             {"CLOUD_MERGE_GATE_MODE": "off"}), \
-                mock.patch("cloud.server._pushback_block",
-                           side_effect=RuntimeError("chain down")), \
+        with mock.patch("cloud.server._pushback_block",
+                        side_effect=RuntimeError("chain down")), \
                 mock.patch("cloud.server._auth_store",
                            side_effect=_auth_store):
             h._merge_task("u1", "t1")