Nice-to-have: CLI wrapper for @rdcp.dev/client (admin utility)

Create a simple CLI wrapper around @rdcp.dev/client to perform discovery, status, control, and health from the terminal (with RDCP_* env support). This was listed as optional in #75.\n\nScope\n- Node CLI that reads RDCP_* env vars for auth\n- Subcommands: discovery, status, control (enable/disable/toggle), health\n- Helpful output for docs\n\nLinkage\n- Follow-up to #75