fix(slack): preserve thread context in auto-replies

When replying to a message in a Slack thread, the response now stays
in the thread instead of going to the channel root.

Only threads replies when the incoming message was already in a thread
(has thread_ts). Top-level messages get top-level replies.

Fixes #250

src/slack/monitor.ts

@@ -738,7 +738,6 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
 
     // Only thread replies if the incoming message was in a thread.
     const incomingThreadTs = message.thread_ts;
-
     const dispatcher = createReplyDispatcher({
       responsePrefix: cfg.messages?.responsePrefix,
       deliver: async (payload) => {