fix: DM responses appearing in separate threads for AI Assistant apps

[bokelley]

Summary

• Fix DM responses appearing as separate threads instead of in the same conversation
• Fix duplicate responses being sent for the same DM message

Problem

Two issues were causing incorrect behavior for Addie's DM responses:

1. Responses appearing in separate threads: For Slack AI/Assistant apps, every DM is treated as a thread. The code was not passing thread_ts when responding, causing responses to appear as separate notifications/threads instead of in the same conversation.

2. Duplicate responses: The Assistant framework receives message.im events through a separate pipeline from global middleware, causing both handleDirectMessage and handleUserMessage to fire for the same event, resulting in duplicate responses with different content.

Changes

• Add thread_ts: event.ts to chat.postMessage in handleDirectMessage to ensure responses appear in the same thread as the user's message
• Add guard in handleUserMessage to skip DMs without thread_ts (these are handled by handleDirectMessage via middleware)

Test plan

• Send a DM to Addie through the App Home Chat tab
• Verify response appears in the same thread as the message (not as a separate notification)
• Verify only one response is sent (no duplicates)
• Send a follow-up message in the same conversation and verify response threading works correctly

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