- Introduce CLI client (cmux) to relay v1 text and v2 JSON-RPC commands over Unix/TCP sockets
- Implement command specs, flag parsing, v1/v2 round-trips, TCP retry with address refresh
- Add browser subcommand mapping and rpc passthrough support
- Support busybox-style invocation when argv[0]=="cmux" and add 'cli' subcommand
- Add comprehensive unit tests for socket dialing, CLI commands, flag-to-param mapping, and env defaults
- Add socket_addr file fallback reader and random request id generation
