NordVPN macOS WireGuard Backend Plan

Add a backend selector to nordvpn-client.
Keep Linux CLI behavior unchanged.
Add macOS WireGuard dependency probing and install guidance.
Implement token-based NordLynx config generation inspired by wg-nord/wgnord.
Replace the current preferred macOS control mode from app-manual to WireGuard when dependencies and token are available.
Keep app-manual as the last fallback only.
Update status, login, connect, disconnect, and verify JSON to expose the backend in use.
Update repo docs and skill docs to reflect the new model and required token/dependencies.
Verify command behavior locally without forcing a live VPN connection unless requested.