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>

CLAUDE.md

@@ -21,8 +21,7 @@ machines when deployed.
   `modules/installer/common.nix` (the auto-installer's own root/nixos login —
   a deliberate, documented choice, see `docs/auto-installer.md`, not
   accidental tech debt) and **SSH public keys** in `variables.nix`
-  (`vars.adminSshKey`, `vars.remoteBuilderAuthorizedKeys`) plus per-host
-  `KEY` values for beszel-agent auth (see `docs/beszel.md`). Don't use the installer's hardcoded hash as a
+  (`vars.adminSshKey`, `vars.remoteBuilderAuthorizedKeys`, `vars.beszelHubKey`). Don't use the installer's hardcoded hash as a
   template for a *real* host — every other host uses sops-nix
   (`hashedPasswordFile`, see "Security Notes" in `README.md`). Flag any *new*
   secret-like string you encounter instead of committing it.