feat: add lazy evaluation for external modules in inventory

- External modules now use 'external' field for lazy evaluation
- Only fetched when building specific host (not during flake check)
- Improves rebuild performance for unrelated hosts
- Update examples and documentation in inventory.nix header

inventory.nix

@@ -1,4 +1,3 @@
-{
 # ============================================================================
 # Fleet Inventory
 # ============================================================================
@@ -44,8 +43,12 @@
 #   athenix.forUser = "username";  # Automatically enables user (sets athenix.users.username.enable = true)
 #
 # External modules (instead of config):
-  #   Device values can be either a config attrset OR a fetchGit/fetchurl call
-  #   that points to an external Nix module. The module will be imported and evaluated.
+#   Device values can be a config attrset with an optional 'external' field:
+#   devices."hostname" = {
+#     external = builtins.fetchGit { ... };  # 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.
 #
 # Examples:
 #   "lab" = { devices = 3; };                           # Quick: lab1, lab2, lab3
@@ -62,11 +65,14 @@
 #     devices."alice".athenix.forUser = "alice123";       # Sets up for user alice123
 #   };
 #   "external" = {
-  #     devices."remote" = builtins.fetchGit {            # External module via Git
+#     devices."remote".external = builtins.fetchGit {     # External module via Git (lazy)
 #       url = "https://github.com/example/config";
 #       rev = "e1ccd7cc3e709afe4f50b0627e1c4bde49165014";
 #     };
-  #   };  # ========== Lab Laptops ==========
+#   };  
+
+{
+  # ========== Lab Laptops ==========
   # Creates: nix-laptop1, nix-laptop2
   # Both get hdh20267 user via overrides
   nix-laptop = {
@@ -120,7 +126,7 @@
           };
         };
       };
-      "usda-dash" = builtins.fetchGit {
+      "usda-dash".external = builtins.fetchGit {
         url = "https://git.factory.uga.edu/MODEL/usda-dash-config.git";
         rev = "dab32f5884895cead0fae28cb7d88d17951d0c12";
         submodules = true;
@@ -146,34 +152,4 @@
   # ========== Ephemeral/Netboot System ==========
   # Creates: nix-ephemeral1
   nix-ephemeral.devices = 1;
-
-  # ========== Example: External Module Configurations ==========
-  # Uncomment to use external modules from Git repositories:
-  #
-  # external-systems = {
-  #   devices = {
-  #     # Option 1: fetchGit with specific revision (recommended for reproducibility)
-  #     "prod-server" = builtins.fetchGit {
-  #       url = "https://github.com/example/server-config";
-  #       rev = "e1ccd7cc3e709afe4f50b0627e1c4bde49165014";  # Full commit hash
-  #       ref = "main";              # Optional: branch/tag name
-  #     };
-  #
-  #     # Option 2: fetchGit with latest from branch (less reproducible)
-  #     "dev-server" = builtins.fetchGit {
-  #       url = "https://github.com/example/server-config";
-  #       ref = "develop";
-  #     };
-  #
-  #     # Option 3: fetchTarball for specific release
-  #     "test-server" = builtins.fetchTarball {
-  #       url = "https://github.com/example/server-config/archive/v1.0.0.tar.gz";
-  #       sha256 = "sha256:0000000000000000000000000000000000000000000000000000";
-  #     };
-  #
-  #     # Option 4: Mix external module with local overrides
-  #     # Note: The external module's default.nix should export a NixOS module
-  #     # that accepts { inputs, ... } as parameters
-  #   };
-  # };
 }