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>

hosts/server/host.nix

@@ -14,7 +14,6 @@
   };
 
   services.beszel.agent.environment = {
-    KEY = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFPR9kwtC4TAeTRu46A7+opZsYpxqkRJ+x/ZyB2GWCeG";
     EXTRA_FILESYSTEMS = "${vars.storageRoot}/${vars.nfsShares.dockerVolumes.subpath}";
     LOG_LEVEL = "debug";
   };