fix(gateway/slack): send tool call progress messages to correct Slack thread

[jneeee]

Problem

Slack tool call progress messages (e.g. ⚙️ read_file: "...") were appearing in the wrong place:

• DM top-level messages: progress messages went to the DM root instead of a thread under the user's message. The final response correctly created a thread via reply_to=ts, but progress messages had no thread anchor.
• Channel @mentions: already worked correctly since source.thread_id = ts (fallback).

Root Cause

In gateway/run.py, _progress_metadata was built solely from source.thread_id:

_progress_metadata = {"thread_id": source.thread_id} if source.thread_id else None

For DM top-level messages, source.thread_id = None (by design — DMs share one session). So _progress_metadata = None, and progress messages were sent without thread_ts to the DM root.

Meanwhile, the final response was sent with reply_to=event.message_id (the original message's ts), which correctly created a thread. The progress messages and final response ended up in different places.

Fix

1. Added event_message_id parameter to _run_agent().
2. _handle_message_with_agent passes event.message_id to _run_agent.
3. _progress_metadata now uses source.thread_id or event_message_id as the thread anchor, matching what reply_to does for the final response.
4. Same fix applied to _status_thread_metadata and stream consumer metadata for consistency.

Tests

Added TestProgressMessageThread in tests/gateway/test_slack.py:

• Verifies DM top-level messages keep source.thread_id = None (session keying unaffected)
• Verifies message_id = ts is available as thread fallback
• Verifies Slack send() passes correct thread_ts when metadata.thread_id is set
• Verifies channel @mention thread assignment still works

[teknium1]

Merged via PR #3063. Cherry-picked onto current main with authorship preserved. Clean fix — thanks for tracking this down!