docs: update documentation for refactored structure

- Update NAMESPACE.md: filesystem.device and swapSize defaults now null
- Update README.md: add 'Using Athenix as a Library' section with lib.mkFleet
- Update copilot-instructions.md: reflect new directory structure and defaults
- Update EXTERNAL_MODULES.md: correct paths from variants/ and glue/ to hw/ and fleet/
- Update PROXMOX_LXC.md: update hardware type path reference

README.md

@@ -54,7 +54,7 @@ users.nix                 # User account definitions
 
 flake.lock                # Locked dependency versions
 
-variants/                 # Hardware type modules (exportable as nixosModules)
+hw/                 # Hardware type modules (exportable as nixosModules)
 ├── default.nix           # Auto-exports all variant types
 ├── nix-desktop.nix       # Desktop workstations
 ├── nix-laptop.nix        # Laptop systems
@@ -64,8 +64,8 @@ variants/                 # Hardware type modules (exportable as nixosModules)
 ├── nix-zima.nix          # ZimaBoard systems
 └── nix-ephemeral.nix     # Diskless/netboot systems
 
-glue/                     # Fleet generation and common configuration
-├── fleet.nix             # Processes inventory.nix to generate all hosts
+fleet/                    # Fleet generation and common configuration
+├── default.nix           # Processes inventory.nix to generate all hosts
 ├── common.nix            # Common NixOS configuration (all hosts)
 ├── boot.nix              # Boot and filesystem configuration
 └── user-config.nix       # User account and home-manager integration
@@ -233,6 +233,35 @@ nix flake show
 
 **See [docs/BUILDING.md](docs/BUILDING.md) for complete guide.**
 
+### Using Athenix as a Library
+
+Import Athenix in your own flake to use its fleet generation logic with custom inventory:
+
+```nix
+{
+  inputs.athenix.url = "git+https://git.factory.uga.edu/UGA-Innovation-Factory/athenix.git";
+  
+  outputs = { self, athenix, ... }: {
+    # Generate configurations with custom fleet and hardware types
+    nixosConfigurations = athenix.lib.mkFleet {
+      fleet = import ./my-inventory.nix;
+      hwTypes = import ./my-hardware-types.nix;
+    };
+    
+    # Or use individual modules
+    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
+      modules = [
+        athenix.nixosModules.hw.nix-desktop  # Use Athenix hardware configs
+        athenix.nixosModules.sw              # Use Athenix software configs
+        ./configuration.nix
+      ];
+    };
+  };
+}
+```
+
+**Exported modules:** `nix-desktop`, `nix-laptop`, `nix-surface`, `nix-lxc`, `nix-wsl`, `nix-ephemeral`, `nix-zima`, `sw`, `common`
+
 ## System Types
 
 Set via `athenix.sw.type`: