feat(slack): tell trigger agent to always reply in the user's thread

[JonasJesus42]

Summary

Follow-up to #439. Even with thread_messages and user_name in the trigger payload, the subscriber had no explicit signal saying which thread_ts to use when replying — and for a fresh top-level DM event.thread_ts is undefined, so the agent could not derive it from the event alone.

Bake the reply contract into both the payload and the trigger definition:

• reply_in_thread_ts = event.thread_ts ?? event.ts is now computed by the publisher. For thread continuations it's the existing thread; for top-level messages it's the message's own ts, which kicks off a new thread the moment the bot replies. The agent never has to decide start-vs-continue.
• reply_instruction: a one-line natural-language directive spelling out the exact SLACK_REPLY_IN_THREAD(channel, thread_ts) tool call, so prompt-driven subscribers see the contract inline.
• Trigger definitions in trigger-store.ts now embed the contract in the description that studio (and any agent reading the schema) sees: "To respond, ALWAYS call SLACK_REPLY_IN_THREAD with channel=channel_id and thread_ts=reply_in_thread_ts so every answer lives inside the user's thread."

Test plan

• In triggerOnly mode, send a top-level DM → reply_in_thread_ts in the payload equals event.ts.
• Reply inside the bot's thread → reply_in_thread_ts equals the thread's thread_ts.
• Trigger subscriber agent uses SLACK_REPLY_IN_THREAD(channel_id, reply_in_thread_ts, ...) and the reply appears in the right thread.
• Studio shows the updated trigger description.

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Summary by cubic

Make Slack agents always reply in the user’s thread. Adds reply_in_thread_ts and a reply_instruction to trigger payloads and documents the contract in trigger descriptions.

• New Features
  • Compute reply_in_thread_ts = event.thread_ts ?? event.ts for slack.message.received and slack.app_mention, so agents never decide start vs continue.
  • Include reply_instruction that spells the exact SLACK_REPLY_IN_THREAD(channel_id, reply_in_thread_ts) call.
  • Update trigger descriptions to document the reply contract and keep all answers in the user’s thread.

^{Written for commit 78bd494. Summary will update on new commits.}