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/cli/STANDALONE.md

@@ -108,6 +108,17 @@ fn task import owner/repo --limit 10 --labels "bug,enhancement"
 ```bash
 fn agent stop <agent-id>            # Stop (pause) a running agent
 fn agent start <agent-id>           # Start (resume) a stopped agent
+fn agent mailbox <agent-id>         # View an agent's mailbox
+```
+
+### Messaging
+
+```bash
+fn message inbox                    # List inbox messages
+fn message outbox                   # List sent messages
+fn message send <agent-id> <msg>    # Send a message to an agent
+fn message read <id>                # Read a specific message
+fn message delete <id>              # Delete a message
 ```
 
 ### Typical workflow