The standalone `_send_slack()` function used by the send_message tool
and cron delivery fallback was not passing `thread_ts` to the Slack API,
causing messages to post to the top-level channel instead of inside
threads.
- Add `thread_ts` parameter to `_send_slack()`
- Include `thread_ts` in the chat.postMessage payload when present
- Pass `thread_id` from `_send_to_platform()` to `_send_slack()`
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
