Implements missing WebSocket event handlers for J5 messaging:
- room:list: List all rooms for the authenticated agent
- message:history: Retrieve paginated message history for a room
Also adds:
- Unit tests for both handlers in socket.test.ts
- E2E validation script test/j5-messaging-validation.js
Completes BARAAA-50 deliverables for real-time messaging.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
