slack-channel-monitor: search lag / @mention / thread / bot identity fixes

Problem

The slack-channel-monitor skill silently drops trigger messages in four real scenarios. All four were observed on a production deployment polling 33 channels once per minute.

Actual Behavior

Deploy the skill and check slack_poller_debug.log, or run the poller manually:

python skills/slack-channel-monitor/scripts/main.py --debug

1. Search index lag: search.messages results are bounded by last_poll, but Slack's search index lags the live stream by ~1-2 min. A trigger message becomes searchable only after last_poll moved past it — dropped forever.
2. @mention triggers unsearchable: search.messages does not index the leading @ of a mention, and a quoted phrase ("openhands") does not match an @openhands mention either. With the default trigger phrase, the search path matches nothing.
3. Thread context lost: search results carry no thread_ts, so a trigger posted as a thread reply looks like a root message and starts a fresh conversation instead of continuing the tracked one.
4. Wrong bot identity with user token: auth.test on a user token returns the human owner, so their own messages are misclassified as bot output and skipped.

Expected Behavior

Trigger messages in a monitored channel reliably start/continue a conversation — whether posted at root level, inside a thread, detected 1-2 min after posting, or when running with a user token.

Acceptance Criteria

• A trigger posted at root level is detected within 1-2 poll runs even though the message is brand-new (search-lag fix).
• An @mention trigger is found by the search path (trigger term searched without the leading @).
• A trigger posted as a thread reply continues the existing tracked conversation, not a new one (thread parent resolved via conversations.replies).
• With SLACK_USER_TOKEN configured, the bot identity is resolved from SLACK_BOT_TOKEN and the human owner's messages are not skipped.
• python -m py_compile skills/slack-channel-monitor/scripts/main.py passes.

Notes

This exact patch has been running in production for ~3 weeks. A PR fixing all four issues is incoming.

Start with skills/slack-channel-monitor/scripts/main.py and reproduce the reported cases using python skills/slack-channel-monitor/scripts/main.py --debug, checking slack_poller_debug.log. Trace search-lag, mention, thread, and token-identity handling against the acceptance criteria, then verify with python -m py_compile skills/slack-channel-monitor/scripts/main.py.