feat(feishu): isolate Harness sessions per topic in topic groups

conversationKey derived the group session key from chat_id alone, so in
Feishu topic groups every topic shared one Harness session: context,
/new, pending approvals, and batch-input state all leaked across topics.

Append the event's thread_id to the session key when present. Topic
groups stamp every message with a thread_id, so each topic now maps to
its own group:<chat_id>🧵<thread_id> session. Regular group chats
carry no thread_id and keep the single shared group:<chat_id> key; p2p
keys are untouched.

- message-utils.mjs: thread-aware conversationKey
- message-utils.test.mjs: topic isolation + regular-group fallback cases
- lib/index.js: regenerated host bundle

src/channels/feishu/message-utils.mjs

@@ -16,6 +16,11 @@ export function conversationKey(event) {
   }
   const chatId = event?.message?.chat_id;
   if (!chatId) throw new Error('Feishu group event has no chat id');
+  // Topic groups: every message belongs to a thread, so key the session per
+  // thread to keep each topic's Harness conversation isolated. Regular group
+  // chats carry no thread_id and keep the single shared `group:<chat_id>` key.
+  const threadId = event?.message?.thread_id;
+  if (typeof threadId === 'string' && threadId.trim()) return `group:${chatId}:thread:${threadId}`;
   return `group:${chatId}`;
 }