Refactor session runtime persistence

tests/test_session_store.py

@@ -80,6 +80,7 @@ class TestStoredAgentSessionRoundTrip(unittest.TestCase):
             'user_context': {'lang': 'en'},
             'system_context': {'os': 'linux'},
             'messages': ({'role': 'user', 'content': 'hi'},),
+            'display_messages': ({'role': 'user', 'content': 'visible hi'},),
             'turns': 3,
             'tool_calls': 7,
             'usage': {'input_tokens': 500, 'output_tokens': 300},
@@ -106,6 +107,7 @@ class TestStoredAgentSessionRoundTrip(unittest.TestCase):
         self.assertEqual(loaded.user_context, session.user_context)
         self.assertEqual(loaded.system_context, session.system_context)
         self.assertEqual(loaded.messages, session.messages)
+        self.assertEqual(loaded.display_messages, session.display_messages)
         self.assertEqual(loaded.turns, session.turns)
         self.assertEqual(loaded.tool_calls, session.tool_calls)
         self.assertEqual(loaded.usage, session.usage)
@@ -157,6 +159,30 @@ class TestStoredAgentSessionRoundTrip(unittest.TestCase):
         self.assertEqual(len(loaded.messages), 2)
         self.assertEqual(loaded.messages[0]['role'], 'user')
         self.assertEqual(loaded.messages[1]['role'], 'assistant')
+        self.assertEqual(loaded.display_messages, loaded.messages)
+
+    def test_load_agent_session_falls_back_to_messages_for_display(self) -> None:
+        with tempfile.TemporaryDirectory() as td:
+            directory = Path(td)
+            path = directory / 'legacy.json'
+            data = {
+                'session_id': 'legacy',
+                'model_config': {},
+                'runtime_config': {'cwd': '/'},
+                'system_prompt_parts': [],
+                'user_context': {},
+                'system_context': {},
+                'messages': [{'role': 'user', 'content': 'legacy hi'}],
+                'turns': 0,
+                'tool_calls': 0,
+                'usage': {},
+                'total_cost_usd': 0.0,
+                'file_history': [],
+            }
+            path.write_text(json.dumps(data))
+            loaded = load_agent_session('legacy', directory=directory)
+
+        self.assertEqual(loaded.display_messages, loaded.messages)
 
     def test_load_defaults_for_missing_optional_fields(self) -> None:
         """Missing optional fields get sensible defaults."""