The Slack reactions handler was processing all reactions unconditionally,
ignoring the reactionNotifications config setting.
This adds the same filtering logic that exists in Telegram/Discord/Signal:
- Check reactionMode config (off/own/all/allowlist)
- Skip bot's own reactions
- For 'own' mode, only process reactions on messages sent by the bot
- For 'allowlist' mode, only process reactions from allowlisted users
Fixes reactions not being routed to agent sessions when configured.
