Refactor session runtime persistence

tests/test_compact.py

@@ -255,6 +255,10 @@ class TestCompactConversation(unittest.TestCase):
         self.assertNotIn('<analysis>', result.summary_text)
         # Summary should contain the actual summary content
         self.assertIn('User wanted to test compaction', result.summary_text)
+        # UI display transcript must remain append-only and not be replaced by
+        # compact_summary / compact_boundary model-context messages.
+        display_contents = [m.content for m in agent.last_session.display_messages]
+        self.assertEqual(display_contents, [m.content for m in msgs])
 
     def test_api_error_returns_compaction_error(self) -> None:
         with tempfile.TemporaryDirectory() as tmp_dir: