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_audit_wiring.py

@@ -601,10 +601,13 @@ class EndpointChainTestCase(unittest.TestCase):
             self._hard_body(), self.owner_key)
         self.assertEqual(status, 200, merged)
 
-    def test_pushback_blocks_even_in_record_mode(self):
+    def test_pushback_blocks_with_full_hard_evidence(self):
+        # A1 改写：旧用例 test_pushback_blocks_even_in_record_mode
+        # 已改名。本用例的性质不变——链上推回阻塞时，即使五项
+        # 证据全齐也 409；只是不再经 record 模式证明“与模式无关”，
+        # 因为服务端已无其它模式可比。
         task = self._completed_task()
         self._block_task(task)
-        os.environ["CLOUD_MERGE_GATE_MODE"] = "record"
         status, blocked = self._req(
             "POST", "/tasks/%s/merge" % task["id"],
             self._hard_body(), self.owner_key)