#12894: feat(whatsapp): add reaction notifications support

## Summary Adds support for WhatsApp reaction notifications. ## Changes - Listen for messages.reaction events from Baileys - Emit system events when reactions are added/removed - Log reaction events for debugging - Cleanup listener on close ## Testing - [x] Build passes <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds WhatsApp reaction notification support by listening to reaction events from the Baileys library. The implementation follows established patterns in the codebase: - Records channel activity for reaction events - Resolves reactor JID to E164 format using existing helper - Enqueues system events for agent notification - Properly logs reaction events for debugging - Correctly cleans up event listeners on connection close The change is isolated to `src/web/inbound/monitor.ts` and integrates cleanly with existing message handling infrastructure. The session key is currently hardcoded with a TODO comment acknowledging the need to resolve via routing in the future. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with low risk - The implementation follows existing patterns in the codebase and is well-contained to a single file. The code properly handles errors, cleans up listeners, and integrates with existing infrastructure. The hardcoded session key is acknowledged with a TODO comment. However, there's a minor issue with type safety where the code uses a type assertion to access `reaction.key.participant` which may not exist in the actual Baileys type definition. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->