Fix flat replies in DMs

DM replies were threaded under Adam's message instead of appearing flat.
Root cause: listener fell back to event["ts"] as thread_ts for new DMs,
causing say() to post as a threaded reply. Removed the fallback so DMs
always use flat messages with placeholder update.

Closes #4

listener/app.py

@@ -64,7 +64,7 @@ def _get_coordination(channel, thread_ts):
 def _dispatch_conversation(text, thread_ts, channel, say, is_dm):
     placeholder_ts = None
     if is_dm:
-        result = say(text="Thinking...", thread_ts=thread_ts)
+        result = say(text="Thinking...")
         placeholder_ts = result["ts"]
 
     lambda_client.invoke(
@@ -143,7 +143,7 @@ def handle_message(event, say):
     if not text:
         return
 
-    thread_ts = event.get("thread_ts") or event["ts"]
+    thread_ts = None
     _dispatch_conversation(text, thread_ts, event["channel"], say, is_dm=True)