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/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...";
   };