refactor: fold host-only modules into the hosts

- argon owns its gpu settings, calibre and the luks drives, now generated from one table
- claudecode.nix is gone, claude-code lives in packages.nix
- home-manager options live next to the home-manager wiring in flake.nix
- settings.nix: one nix block, extra-substituters instead of repeating the cache.nixos.org defaults
- README: current layout

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -6,20 +6,20 @@ My personal NixOS configurations.
 
 ```bash
 .
-├── flake.lock
-├── flake.nix # flake definition
+├── flake.nix # hosts and home-manager wiring
 ├── hosts
-│   └── neon
-│       ├── configuration.nix # import nix-modules for specific host
-│       └── hardware-configuration.nix # hardware configs for specific host
+│   ├── argon # desktop
+│   ├── neon # laptop
+│   └── default # test vm
+│       ├── configuration.nix # host-specific settings and module imports
+│       ├── hardware-configuration.nix # generated by nixos-generate-config
+│       └── niri.nix # host-specific niri settings (outputs, input)
 ├── modules
-│   ├── home-manager
-│   │   ├── xyz_module.nix
-│   └── nixos
-│       └── xyz_module.nix
+│   ├── home-manager # user modules, imported by users/aaron/home.nix
+│   └── nixos # system modules, imported by the hosts
 └── users
     └── aaron
-        └── home.nix # import home-manager modules for specific user
+        └── home.nix # imports home-manager modules
 ```
 
 ## installation