fix: Lazily fetch external modules only if needed

users.nix

@@ -13,8 +13,9 @@
   #
   # External User Configuration:
   # Users can specify external configuration modules via the 'external' attribute:
-  #   external = builtins.fetchGit { url = "..."; rev = "..."; };
+  #   external = { url = "..."; rev = "..."; submodules? = false; };
   #   external = /path/to/local/config;
+  #   external = builtins.fetchGit { ... }; # legacy, still supported
   #
   # External repositories should contain:
   #   - user.nix (required): Defines athenix.users.<name> options AND home-manager config
@@ -47,9 +48,10 @@
       enable = true; # Default user, enabled everywhere
     };
     hdh20267 = {
-      external = builtins.fetchGit {
+      external = {
         url = "https://git.factory.uga.edu/hdh20267/hdh20267-nix";
         rev = "dbdf65c7bd59e646719f724a3acd2330e0c922ec";
+        # submodules = false; # optional, defaults to false
       };
     };
     sv22900 = {