refactor: use direct tailscale auth-key enrollment

Stop writing auth keys to guest files and enroll nodes by running tailscale up directly via Proxmox guest agent with VM-name hostnames.

nixos/template-base/configuration.nix

@@ -39,37 +39,6 @@
 
   security.sudo.wheelNeedsPassword = false;
 
-  systemd.services.tailscale-firstboot = {
-    description = "One-time Tailscale enrollment";
-    after = [ "network-online.target" "tailscaled.service" ];
-    wants = [ "network-online.target" "tailscaled.service" ];
-    wantedBy = [ "multi-user.target" ];
-    serviceConfig = {
-      Type = "oneshot";
-      RemainAfterExit = true;
-    };
-    script = ''
-      install -d -m 0700 /etc/tailscale
-
-      if [ ! -s /etc/tailscale/authkey ]; then
-        exit 0
-      fi
-
-      key="$(cat /etc/tailscale/authkey)"
-      ts_hostname=""
-      if [ -s /etc/tailscale/hostname ]; then
-        ts_hostname="--hostname=$(cat /etc/tailscale/hostname)"
-      fi
-
-      install -d -m 0700 /var/lib/tailscale
-      rm -f /var/lib/tailscale/tailscaled.state
-      ${pkgs.tailscale}/bin/tailscale up --reset --auth-key="$key" $ts_hostname
-
-      rm -f /etc/tailscale/authkey
-      rm -f /etc/tailscale/hostname
-    '';
-  };
-
   environment.systemPackages = with pkgs; [
     btop
     curl