Handle indirect Slack review triggers

[DipeshRajoria007]

Summary

• register actionable bot_message events instead of dropping them unconditionally
• detect configured core-dev Slack user-group mentions as indirect triggers
• preserve a stable actor id for bot-authored Slack events and keep config test fixtures aligned with the current settings schema

Root cause

A Slack post like <!subteam^...> Please review and merge <PR> from Friday was skipped in two places:

1. intake ignored every bot_message
2. mention detection only recognized direct bot mentions and owner user mentions, not configured Slack user groups

Testing

• npm --prefix sidecar run test
• npm --prefix sidecar run build