feat(FN-989): add inter-agent messaging system with mailbox UI and CLI commands

- Add Message types (Message, MessageThread, MessageRecipient) and exports to @fusion/core
- Create MessageStore with full CRUD: send, read, delete, inbox, threads, and search
- Add messages table migration (schema v12) with SQLite full-text search support
- Add REST API routes for messaging (CRUD, search, broadcast, unread count)
- Add frontend API client functions for all messaging endpoints
- Build MailboxModal and MessageComposer dashboard components with header integration
- Add CLI message commands (inbox, send, read, delete) with rich output formatting
- Add comprehensive test coverage for MessageStore, CLI commands, and UI components
- Update documentation (CLI STANDALONE.md, dashboard README) with messaging usage

packages/dashboard/README.md

@@ -529,6 +529,18 @@ When `FUSION_BADGE_PUBSUB_REDIS_URL` is not set, the dashboard uses an in-memory
 - `DELETE /api/terminal/sessions/:id` - Kill session
 - `WS /api/terminal/ws` - WebSocket connection
 
+### Messaging
+- `GET /api/messages/inbox` - Fetch inbox messages (query: `limit`, `offset`, `unreadOnly`, `type`)
+- `GET /api/messages/outbox` - Fetch sent messages (query: `limit`, `offset`, `type`)
+- `GET /api/messages/unread-count` - Get unread count (for header badge)
+- `POST /api/messages` - Send a message (body: `{ toId, toType, content, type, metadata? }`)
+- `GET /api/messages/:id` - Fetch a single message
+- `POST /api/messages/:id/read` - Mark message as read
+- `POST /api/messages/read-all` - Mark all inbox as read
+- `DELETE /api/messages/:id` - Delete a message
+- `GET /api/messages/conversation/:participantType/:participantId` - Get conversation thread
+- `GET /api/agents/:id/mailbox` - View agent mailbox (admin read-only)
+
 ### Configuration
 - `GET /api/config` - Server configuration
 - `GET /api/settings` - Merged settings (project overrides global)