refactor(beszel): switch to universal token via secrets/common.yaml

Replace per-host host-token.nix imports with a single beszel-token secret
in secrets/common.yaml, wired once in enable-agent.nix. Host files now only
need services.beszel.agent.environment.KEY — no imports block required.

Delete modules/beszel/host-token.nix (no longer referenced anywhere).

Action needed: run `sops secrets/common.yaml` and add `beszel-token: <value>`
from the beszel hub UI before deploying.

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

hosts/nix-cache/host.nix

@@ -1,13 +1,6 @@
 { vars, ... }:
 
 {
-  imports = [
-    (import ../../modules/beszel/host-token.nix {
-      name = "nix-cache";
-      sopsFile = ../../secrets/nix-cache.yaml;
-    })
-  ];
-
   networking = {
     hostName = vars.nixCacheHost;
     useDHCP = false;