Add lxc-tor-relay build type with nyx monitoring

New tor-relay build type (currently lxc-only) running a plain Tor
middle relay via modules/tor/enable-relay.nix, plus nyx for
interactive monitoring over the relay's control socket.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/auto-installer.md

@@ -19,7 +19,7 @@ see "LXC hosts" immediately below for why those are different.**
 ## LXC hosts
 
 `lxc-*` targets (`lxc-minimal`, `lxc-nix-cache`, `lxc-server`, `lxc-docker`,
-`lxc-gui`, `lxc-pxe-boot`, `lxc-tailscale-exit-node`) are **not** installed via `auto-install.sh` — the
+`lxc-gui`, `lxc-pxe-boot`, `lxc-tailscale-exit-node`, `lxc-tor-relay`) are **not** installed via `auto-install.sh` — the
 interactive menu deliberately excludes them. Don't try to select one there;
 `nixos-install` would bind-mount `/` onto `/mnt` (LXC containers have no raw
 disk to partition) and then refuse to touch the filesystem it's currently