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/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