feat(beszel): add beszel agent to tailscale-router

Wires beszel-agent into all tailscale-router variants (lxc/linode/proxmox)
by importing enable-agent.nix in the build type and host-token.nix in the
host file. Adds the sops creation rule for secrets/tailscale-router.yaml
(all three platform variants as recipients). The secrets file must be
created manually before deploying — see instructions in PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/build-types/tailscale-router.nix

@@ -3,6 +3,7 @@
 {
   imports = [
     ../tailscale/subnet-router.nix
+    ../beszel/enable-agent.nix
   ];
 
   # "server", not "both": this build type advertises LAN subnet routes but