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/tor-relay/host.nix

@@ -15,10 +15,6 @@
   # No networking.hostId: only ZFS-touching hosts need one for pool-import
   # safety, and this host does neither.
 
-  services.beszel.agent.environment = {
-    KEY = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFPR9kwtC4TAeTRu46A7+opZsYpxqkRJ+x/ZyB2GWCeG";
-  };
-
   # A genuinely new host (not a pre-refactor carry-over), so it tracks the
   # flake's current nixpkgs release rather than being pinned to an older one.
   system.stateVersion = "26.05";