feat: add message burst debouncing (MessageBatcher)

- Create MessageBatcher: debounce 600ms, adaptive delay for long chunks,
  max 8 messages/4000 chars before immediate flush, per (chatId, sender) key
- Integrate into trigger: text messages enqueued, slash commands and
  file/image bypass batching, locked projects respond immediately
- Add unit tests (8) for batcher lifecycle and edge cases

hub/test/unit/feishu-reactions.test.ts

@@ -103,6 +103,7 @@ async function triggerWithRunAgent(
     logger: rt.logger,
     authorizer: allowAllAuthorizer(),
     runAgent,
+    messageBatcherOptions: { maxMessages: 1 },
   });
 
   await trigger(makeEvent(), rt);