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

README.md

@@ -54,18 +54,21 @@ users.nix                 # User account definitions
 
 flake.lock                # Locked dependency versions
 
-hosts/                    # Host generation logic
-├── default.nix           # Main host generator
+variants/                 # Hardware type modules (exportable as nixosModules)
+├── default.nix           # Auto-exports all variant types
+├── nix-desktop.nix       # Desktop workstations
+├── nix-laptop.nix        # Laptop systems
+├── nix-surface.nix       # Surface Pro tablets
+├── nix-lxc.nix           # LXC containers
+├── nix-wsl.nix           # WSL instances
+├── 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
+├── common.nix            # Common NixOS configuration (all hosts)
 ├── boot.nix              # Boot and filesystem configuration
-├── common.nix            # Common system configuration
-├── user-config.nix       # User configuration integration
-└── types/                # Hardware type modules
-    ├── nix-desktop.nix
-    ├── nix-laptop.nix
-    ├── nix-surface.nix
-    ├── nix-lxc.nix
-    ├── nix-wsl.nix
-    └── nix-ephemeral.nix
+└── user-config.nix       # User account and home-manager integration
 
 sw/                       # Software configurations by system type
 ├── default.nix           # Software module entry point