Add `unarr vpn` (status/enable/disable, with `status --check`) to manage the
managed WireGuard split-tunnel from the CLI. The daemon now reports its
split-tunnel state (active, mode, exit server) to the web on register and on
every sync, and sends its agent id when fetching the VPN config so the web can
arbitrate the single WireGuard slot (1 VPNResellers account = 1 WG keypair = 1
concurrent connection): the first agent claims it; the rest are told to run
OpenVPN on their own host (1 WireGuard + up to 9 OpenVPN = 10).
`status --check` passes probe=1 so it validates provisioning without claiming
the slot. VPNActive drops omitempty so a downed tunnel reaches the server and
frees the slot. Bumps to 0.9.2 with CHANGELOG + README VPN section.
