fix(slack): pass thread_ts in standalone send_message tool path

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>

tools/send_message_tool.py

@@ -767,7 +767,7 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
     last_result = None
     for chunk in chunks:
         if platform == Platform.SLACK:
-            result = await _send_slack(pconfig.token, chat_id, chunk)
+            result = await _send_slack(pconfig.token, chat_id, chunk, thread_ts=thread_id)
         elif platform == Platform.WHATSAPP:
             result = await _send_whatsapp(pconfig.extra, chat_id, chunk)
         elif platform == Platform.SIGNAL:
@@ -1049,7 +1049,7 @@ async def _send_telegram(token, chat_id, message, media_files=None, thread_id=No
         return _error(f"Telegram send failed: {e}")
 
 
-async def _send_slack(token, chat_id, message):
+async def _send_slack(token, chat_id, message, thread_ts=None):
     """Send via Slack Web API."""
     try:
         import aiohttp
@@ -1063,6 +1063,8 @@ async def _send_slack(token, chat_id, message):
         headers = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}
         async with aiohttp.ClientSession(timeout=aiohttp.ClientTimeout(total=30), **_sess_kw) as session:
             payload = {"channel": chat_id, "text": message, "mrkdwn": True}
+            if thread_ts:
+                payload["thread_ts"] = thread_ts
             async with session.post(url, headers=headers, json=payload, **_req_kw) as resp:
                 data = await resp.json()
                 if data.get("ok"):