fix(session): serialize direct persistence flushes

run_agent.py

@@ -1765,6 +1765,18 @@ class AIAgent:
         return repair_message_sequence(self, messages)
 
     def _flush_messages_to_session_db(self, messages: List[Dict], conversation_history: List[Dict] = None):
+        """Serialize direct and turn-boundary session flushes per agent."""
+        persist_lock = getattr(self, "_session_persist_lock", None)
+        if persist_lock is None:
+            return self._flush_messages_to_session_db_unlocked(messages, conversation_history)
+        with persist_lock:
+            return self._flush_messages_to_session_db_unlocked(messages, conversation_history)
+
+    def _flush_messages_to_session_db_unlocked(
+        self,
+        messages: List[Dict],
+        conversation_history: List[Dict] = None,
+    ):
         """Persist any un-flushed messages to the SQLite session store.
 
         Deduplicates via an intrinsic ``_DB_PERSISTED_MARKER`` stamped on each