fix: Lazily fetch external modules only if needed

inventory.nix

@@ -45,7 +45,7 @@
 # External modules (instead of config):
 #   Device values can be a config attrset with an optional 'external' field:
 #   devices."hostname" = {
-#     external = builtins.fetchGit { ... };  # Lazy: only fetched when building this host
+#     external = { url = "..."; rev = "..."; submodules? = false; };  # Lazy: only fetched when building this host
 #     # ... additional config options
 #   };
 #   The external module will be imported and evaluated only when this specific host is built.
@@ -65,7 +65,7 @@
 #     devices."alice".athenix.forUser = "alice123";       # Sets up for user alice123
 #   };
 #   "external" = {
-#     devices."remote".external = builtins.fetchGit {     # External module via Git (lazy)
+#     devices."remote".external = { url = "..."; rev = "..."; };  # External module via Git (lazy)
 #       url = "https://github.com/example/config";
 #       rev = "e1ccd7cc3e709afe4f50b0627e1c4bde49165014";
 #     };
@@ -127,7 +127,7 @@
             };
           };
         };
-        "usda-dash".external = builtins.fetchGit {
+        "usda-dash".external = {
           url = "https://git.factory.uga.edu/MODEL/usda-dash-config.git";
           rev = "dab32f5884895cead0fae28cb7d88d17951d0c12";
           submodules = true;