fix: isolate review-mutation ledger from host session-state (#590)

Tests that call patch.dict(os.environ, clear=True) dropped
GITEA_MCP_SESSION_STATE_DIR and re-adopted the operator host cache
under ~/.cache/gitea-tools/session-state. A residual terminal
request_changes mutation then tripped the #332 hard-stop before
test_unknown_profile_blocks could assert the empty-profile gate.

- Pin mcp_session_state.default_state_dir / DEFAULT_STATE_DIR to a
  per-test temp dir in conftest (still honors an explicit env override)
- Clear the decision lock at the start of each TestMergePR test
- Add regression coverage for env-clear isolation

Closes #590

tests/test_mcp_server.py

@@ -774,6 +774,9 @@ class TestMergePR(unittest.TestCase):
     def setUp(self):
         import reviewer_pr_lease
 
+        # Start each merge test with a clean review-mutation ledger (#590).
+        # Host durable state can otherwise leak in when tests clear os.environ.
+        mcp_server._save_review_decision_lock(None)
         mcp_server.gitea_load_review_workflow()
         self._lease_patch = _install_owned_reviewer_lease(8)
         self._lease_patch.start()
@@ -1003,6 +1006,64 @@ class TestMergePR(unittest.TestCase):
             r["reasons"])
         self._assert_no_merge_call(mock_api)
 
+    @patch("mcp_server.api_request")
+    @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
+    def test_env_clear_does_not_adopt_host_review_mutation_ledger(
+        self, _auth, mock_api
+    ):
+        """#590: patch.dict(clear=True) must not load host terminal mutations.
+
+        Simulates a dirty host session-state file for profile gitea-default
+        (the profile active when env is fully cleared). Isolation must keep
+        the #332 hard-stop ledger clean so the empty-profile gate fires.
+        """
+        import tempfile
+
+        import mcp_session_state
+
+        poison_dir = tempfile.mkdtemp(prefix="gitea-host-poison-")
+        mcp_session_state.save_state(
+            kind=mcp_session_state.KIND_DECISION_LOCK,
+            profile_identity="gitea-default",
+            state_dir=poison_dir,
+            payload={
+                "task": "review_pr",
+                "remote": "prgs",
+                "final_review_decision_ready": True,
+                "ready_pr_number": 8,
+                "ready_action": "request_changes",
+                "live_mutations": [
+                    {
+                        "pr_number": 8,
+                        "action": "request_changes",
+                        "review_id": 99,
+                        "review_state": "request_changes",
+                    }
+                ],
+            },
+        )
+        # If isolation only used the env var, clear=True would fall back to
+        # DEFAULT_STATE_DIR and adopt this poison ledger.
+        mcp_server._REVIEW_DECISION_LOCK = None
+        mock_api.side_effect = [{"login": "merger-bot"}, self._pr("author-bot")]
+        with patch.object(mcp_session_state, "DEFAULT_STATE_DIR", poison_dir):
+            with patch.dict(os.environ, {}, clear=True):
+                r = gitea_merge_pr(
+                    pr_number=8,
+                    confirmation=self._confirm(8),
+                    remote="prgs",
+                )
+        self.assertFalse(r["performed"])
+        self.assertIn(
+            "profile has no configured allowed operations (fail closed)",
+            r["reasons"],
+        )
+        self.assertFalse(
+            any("terminal review mutation already consumed" in x for x in r["reasons"]),
+            msg=f"host ledger leaked into reasons: {r['reasons']}",
+        )
+        self._assert_no_merge_call(mock_api)
+
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
     def test_profile_without_merge_permission_blocks(self, _auth, mock_api):