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

docs/EXTERNAL_MODULES.md

@@ -115,8 +115,8 @@ server-config/
 
 When a host is built, modules load in this order:
 
-1. Hardware type module (from `variants/nix-*.nix`)
-2. Common system configuration (from `glue/common.nix`)
+1. Hardware type module (from `hw/nix-*.nix`)
+2. Common system configuration (from `fleet/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/NAMESPACE.md

@@ -17,9 +17,9 @@ Hardware and boot-related settings.
 
 Boot disk device path.
 
-**Type:** String
+**Type:** String or null
 
-**Default:** `"/dev/sda"`
+**Default:** `null` (must be set by hardware type or per-host)
 
 **Example:**
 ```nix
@@ -30,9 +30,9 @@ athenix.host.filesystem.device = "/dev/nvme0n1";
 
 Swap partition size.
 
-**Type:** String (size with unit, e.g., `"32G"`, `"2G"`)
+**Type:** String (size with unit, e.g., `"32G"`, `"2G"`) or null
 
-**Default:** `"32G"`
+**Default:** `null` (must be set by hardware type or per-host)
 
 **Example:**
 ```nix