Add diagnostic and setup commands so users can verify their environment
and install greyproxy without leaving greywall:
- `greywall check`: shows version, platform deps, security features,
and greyproxy installation/running status (absorbs old --version output)
- `greywall setup`: downloads greyproxy from GitHub releases and shells
out to `greyproxy install`, or auto-starts if already installed
- `--version` simplified to single-line output for scripting
New `internal/proxy/` package handles greyproxy detection (LookPath +
/api/health endpoint), GitHub release fetching, tar.gz extraction,
and service lifecycle management.
