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...";
   };