refactor(beszel): move KEY and TOKEN fully into enable-agent.nix

KEY (hub's SSH public key) is now stored in variables.nix as beszelHubKey
and set by enable-agent.nix — no host file ever needs to touch it.
TOKEN was already universal; this removes the last per-host beszel config.

Importing enable-agent.nix in a build type is now the only step needed to
add a new host to beszel monitoring.

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

modules/beszel/enable-agent.nix

@@ -16,6 +16,7 @@
     environment = {
       #DOCKER_HOST = "tcp://docker-socket-proxy:2375";
       HUB_URL = "http://${vars.dockerHost}.${vars.homeDomain}:${toString vars.ports.beszelHub}";
+      KEY = vars.beszelHubKey;
     };
   };