feat(guard): native MCP transport binding and contaminated-review quarantine (Closes #695)

Bind mutation/credential paths to a process-local native MCP runtime so env
spoofing, direct imports, and offline helpers cannot reconstruct session gates
after native transport failure. Quarantine contaminated formal reviews under
controller authority and honor quarantine in review feedback, merge eligibility,
merge mutation, and canonical handoff validation. Add regression coverage for
the second (PR #694 / review 427) incident class.

tests/test_merge_approval_gate.py

@@ -54,6 +54,26 @@ class TestMergeApprovalGate(unittest.TestCase):
         self.assertFalse(result["approval_at_current_head"])
         self.assertIsNone(result["latest_approved_head_sha"])
 
+    def test_quarantined_approval_void_for_merge(self):
+        """#695: contaminated formal review at head must not authorize merge."""
+        result = assess_merge_approval_head(
+            current_head_sha=HEAD_NEW,
+            latest_by_reviewer={
+                "sysadmin": {
+                    "verdict": "APPROVED",
+                    "dismissed": False,
+                    "reviewed_head_sha": HEAD_NEW,
+                    "review_id": 427,
+                    "submitted_at": "2026-07-13T07:20:00Z",
+                }
+            },
+            quarantined_review_ids={427},
+        )
+        self.assertFalse(result["approval_at_current_head"])
+        self.assertEqual(result["quarantined_approvals_at_current_head"], 1)
+        self.assertIn("quarantined", result["stale_approval_block_reason"])
+        self.assertIn("#695", result["stale_approval_block_reason"])
+
 
 if __name__ == "__main__":
     unittest.main()