Extract ports, docker host, and remote-builder keys into variables.nix

Pulls the beszel hub / PVE / PBS ports, the docker-compose host's LAN
name, and the remote-builder client SSH keys out of scattered inline
literals across modules/hosts and into variables.nix as the single
source of truth, matching the existing pattern for other cross-host
references (nixCacheHost, nfsServerHost).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

modules/beszel/enable-agent.nix

@@ -4,6 +4,6 @@
   services.beszel.agent.enable = true;
   services.beszel.agent.environment = {
     #DOCKER_HOST = "tcp://docker-socket-proxy:2375";
-    HUB_URL = "http://docker.${vars.homeDomain}:8090";
+    HUB_URL = "http://${vars.dockerHost}.${vars.homeDomain}:${toString vars.beszelHubPort}";
   };
 }