feat(ai-tools): add enable option

modules/ai-tools/default.nix

@@ -1,11 +1,14 @@
-{ config, ... }:
+{ lib, config, ... }:
 
 let
+  cfg = config."ai-tools";
   inherit (config.host) username;
   inherit (config.secrets) owner;
 in
 {
-  config = {
+  options."ai-tools".enable = lib.mkEnableOption "AI tools";
+
+  config = lib.mkIf cfg.enable {
     nixpkgs.allowedUnfree = [ "claude-code" ];
     secrets.groups.opencode = [ "api-key" ];