Prevent duplicate Twitch session prompts

app/agent/policies.py

@@ -27,6 +27,10 @@ class ChatActivityPolicy:
         """Get the most recent chat activity time for a session."""
         return self.last_message_time.get(session_id)
 
+    def clear_activity(self, session_id: str) -> None:
+        """Stop tracking activity for an ended session."""
+        self.last_message_time.pop(session_id, None)
+
     def minutes_since_activity(self, session_id: str) -> int:
         """Get minutes since last chat message."""
         if session_id not in self.last_message_time: