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>

tests/tools/test_send_message_tool.py

@@ -595,6 +595,7 @@ class TestSendToPlatformChunking:
             "***",
             "C123",
             "*hello* from <https://example.com|Hermes>",
+            thread_ts=None,
         )
 
     def test_slack_bold_italic_formatted_before_send(self, monkeypatch):