docs: update all references from hosts/ to glue/ and variants/

- Update README.md structure section
- Update DEVELOPMENT.md, EXTERNAL_MODULES.md, INVENTORY.md
- Update GitHub Copilot instructions
- Update PROXMOX_LXC.md references
- Clarify new directory organization and purpose

docs/DEVELOPMENT.md

@@ -87,7 +87,7 @@ athenix.users.newuser = {
 };
 ```
 
-2. Enable on hosts in `inventory.nix`:
+2. Enable on fleet in `inventory.nix`:
 
 ```nix
 nix-laptop = {
@@ -294,7 +294,7 @@ For system config:
 ```nix
 # inventory.nix
 nix-lxc = {
-  devices."server" = builtins.fetchGit {
+  devices."server".external = builtins.fetchGit {
     url = "https://git.factory.uga.edu/org/server-config";
     rev = "abc123...";
   };

docs/EXTERNAL_MODULES.md

@@ -28,26 +28,42 @@ External system modules provide host-specific NixOS configurations.
 
 ### Usage
 
-In `inventory.nix`, reference an external module as a device:
+In `inventory.nix`, reference an external module using the `external` field:
 
 ```nix
 nix-lxc = {
   devices = {
-    # Inline configuration
+    # Inline configuration (traditional method)
     "local-server" = {
       athenix.sw.type = "headless";
       services.nginx.enable = true;
     };
     
-    # External module
-    "remote-server" = builtins.fetchGit {
+    # External module (lazy evaluation - fetched only when building this host)
+    "remote-server".external = builtins.fetchGit {
       url = "https://git.factory.uga.edu/org/server-config";
       rev = "abc123def456...";  # Must pin to specific commit
     };
+    
+    # External module with additional local config
+    "mixed-server" = {
+      external = builtins.fetchGit {
+        url = "https://git.factory.uga.edu/org/server-config";
+        rev = "abc123def456...";
+      };
+      # Additional local overrides
+      athenix.users.admin.enable = true;
+      services.openssh.permitRootLogin = "no";
+    };
   };
 };
 ```
 
+**Key Features:**
+- **Lazy Evaluation**: External modules are only fetched when building the specific host
+- **Efficient Rebuilds**: Other hosts can be rebuilt without fetching unrelated external modules
+- **Submodule Support**: Works with Git submodules without affecting other hosts
+
 ### Repository Structure
 
 ```
@@ -99,8 +115,8 @@ server-config/
 
 When a host is built, modules load in this order:
 
-1. Hardware type module (from `hosts/types/nix-*.nix`)
-2. Host common configuration (from `hosts/common.nix`)
+1. Hardware type module (from `variants/nix-*.nix`)
+2. Common system configuration (from `glue/common.nix`)
 3. Software type module (from `sw/{type}/`)
 4. User NixOS modules (from `users.nix` - `nixos.nix` files)
 5. Device-specific overrides (from `inventory.nix`)

docs/INVENTORY.md

@@ -123,11 +123,11 @@ nix-surface = {
 
 ### Method 4: External Module
 
-Reference a Git repository instead of inline configuration:
+Reference a Git repository using the `external` field (lazy evaluation):
 
 ```nix
 nix-lxc = {
-  devices."builder" = builtins.fetchGit {
+  devices."builder".external = builtins.fetchGit {
     url = "https://git.factory.uga.edu/org/builder-config";
     rev = "abc123...";
   };