fix: support Slack thread delivery targets

[EndeavorYen]

Summary

• allow explicit Slack send_message targets to include a thread_ts suffix, e.g. slack:D123ABCDEF:1777740958.516299
• pass the parsed Slack thread_ts through to chat.postMessage
• add cron/send_message regression coverage for Slack thread delivery targets
• isolate silent-delivery cron tests from the shared scheduler lock under xdist

Why

Telegram and Discord delivery targets already support platform:chat_id:thread_id, but Slack explicit conversation IDs were parsed as channel-only targets. That means a cron or tool delivery target like slack:D...:thread_ts could be treated as an invalid Slack channel value instead of posting into the intended thread.

Validation

• /Users/simon/.hermes/hermes-agent/venv/bin/python -m pytest tests/tools/test_send_message_tool.py::TestParseTargetRefSlack tests/tools/test_send_message_tool.py::TestSendToPlatformChunking::test_slack_thread_id_is_passed_to_send tests/cron/test_scheduler.py::TestResolveDeliveryTarget
• /Users/simon/.hermes/hermes-agent/venv/bin/python -m pytest tests/tools/test_send_message_tool.py tests/cron/test_scheduler.py

Local runtime smoke on a forked Hermes deployment also confirmed cron delivery posted into the expected Slack thread.

[alt-glitch]

Likely duplicate of #16992 — same fix for Slack thread_ts parsing in send_message targets. Also related to earlier closed #14146 and #16962.

[alt-glitch]

Likely duplicate of #16992

[EndeavorYen]

Thanks, agreed. #16992 covers the main runtime fix here: parsing Slack thread delivery targets and passing thread_ts through to chat.postMessage.

I’ll close this PR to avoid splitting the same fix across two branches. The only extra pieces here are:

• a cron delivery-target regression test for slack:conversation_id:thread_ts
• xdist isolation for TestSilentDelivery, which was sharing the scheduler lock
• slightly stricter Slack conversation ID validation (C/G/D...) for explicit targets

Happy to move any of those into #16992 if useful.