feat(nixos): encapsulate CachyOS kernel configuration

configs/nixos/hosts/zeus/default.nix

@@ -1,7 +1,6 @@
 {
   config,
   lib,
-  pkgs,
   ...
 }:
 # zeus — Ryzen desktop with AMD graphics.
@@ -15,7 +14,7 @@
 
   nixpkgs.hostPlatform = "x86_64-linux";
 
-  boot.kernelPackages = pkgs.linuxPackages_cachyos;
+  modules.kernel.cachyos.enable = true;
 
   hardware.graphics = {
     enable = true;