fix: flat DM replies, clean up debug logging

DMs now reply flat in conversation instead of threading. Channel
@mentions still thread. Removed verbose debug logging from socket-mode
service, kept minimal operational logs. Added .DS_Store to gitignore.

.gitignore

@@ -22,3 +22,6 @@ docs/
 
 # Claude Code project settings
 .claude/
+
+# macOS
+.DS_Store

lambda/slack-processor/index.ts

@@ -173,12 +173,14 @@ export const handler = async (event: ProcessorEvent): Promise<void> => {
     resolveSessionId(userId),
   ]);
 
-  // Post placeholder immediately so the user sees a response while the agent thinks
+  // In channels, reply in a thread; in DMs, reply flat (no thread_ts)
+  const replyThreadTs = eventType === 'app_mention' ? (threadTs ?? ts) : threadTs;
+
   const placeholderTs = await postToSlack(
     credentials.botToken,
     channelId,
     '_Alex is thinking..._',
-    threadTs ?? ts,
+    replyThreadTs,
   );
 
   let answer: string;

services/socket-mode/src/index.ts

@@ -36,6 +36,7 @@ async function main(): Promise<void> {
     if (event.bot_id || event.subtype) return;
     if (event.channel_type !== 'im') return;
 
+    console.log('Processing DM from', event.user);
     await invokeProcessor({
       userId: event.user,
       channelId: event.channel,
@@ -51,6 +52,7 @@ async function main(): Promise<void> {
 
     const text = (event.text ?? '').replace(/^<@[A-Z0-9]+>\s*/, '');
 
+    console.log('Processing @mention from', event.user, 'in', event.channel);
     await invokeProcessor({
       userId: event.user,
       channelId: event.channel,