Add reusable clean pair runner

tests/test_core_flow.py

@@ -4845,6 +4845,18 @@ class CoreFlowTests(unittest.TestCase):
             self.assertTrue(proposal_path.exists())
             proposal = json.loads(proposal_path.read_text(encoding="utf-8"))
             self.assertTrue(proposal["should_stop"])
+            state = store.load_state(study.study_id)
+            self.assertEqual(state.tuning_stop_reason, "harness_stop")
+            self.assertEqual(
+                state.tuning_stop_details["proposal_name"],
+                "harness-stop-0005",
+            )
+            self.assertEqual(state.tuning_stop_details["proposal_source"], "harness")
+            self.assertEqual(
+                state.tuning_stop_details["stop_authorized_by"],
+                "validator",
+            )
+            self.assertTrue(state.tuning_stop_diagnosis)
 
     def test_cli_tune_evaluates_baseline_before_llm_proposal(self) -> None:
         with tempfile.TemporaryDirectory() as tmp: