fix(slack): support thread targets in send_message

[bejewel-sangwon]

Summary

• Parse Slack send_message targets with an optional thread timestamp, e.g. slack:GK0J1C6LT:1778502321.731699.
• Pass the parsed thread timestamp through to Slack chat.postMessage as thread_ts.
• Add regression tests for Slack thread target parsing and sender propagation.

Test Plan

• uv run --extra dev pytest tests/tools/test_send_message_tool.py

[alt-glitch]

Duplicate of #16992 — same Slack thread target parsing fix (channel_id:thread_ts format).

[teknium1]

This looks implemented on current main now. Automated hermes-sweeper review.

Evidence:

• tools/send_message_tool.py:31 defines Slack thread targets as <conversation_id>:<thread_ts> via _SLACK_THREAD_TARGET_RE.
• tools/send_message_tool.py:369 parses that Slack thread target and returns (chat_id, thread_id, True).
• tools/send_message_tool.py:784 forwards thread_id into _send_slack(..., thread_ts=thread_id).
• tools/send_message_tool.py:1064 / tools/send_message_tool.py:1079 accepts thread_ts and includes it in the Slack chat.postMessage payload.
• Regression coverage exists in tests/tools/test_send_message_tool.py:312 for Slack thread target propagation.

The parse side landed in 2f28b60a474c880367be612c682f52b8ca9dbb4d, and the Slack API payload propagation landed in 74e845c000de1f32cd325758407ea706f18b7c36, which is contained in v2026.6.5. The prior maintainer note marking this as a duplicate of #16992 matches the implementation now present on main.