feat(chat): support chat resources, implement chat-based remote discovery via resource messages

backend/src/app_mixins/chat.py

@@ -152,6 +152,10 @@ class ChatMixin:
         # Notification order depends on whether the content is a stream
 
         async def notify_agents():
+            # Not all messages are visible to agents
+            if message.kind == "resource":
+                return
+
             # Fetch participant agents
             if tmp.is_tmp_id(chat_id):
                 agent_ids = self.tmp.chats[chat_id].participants