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>

hosts/neon/configuration.nix

@@ -5,7 +5,6 @@
     ../../modules/nixos/audio.nix
     ../../modules/nixos/bootloader.nix
     ../../modules/nixos/certificates.nix
-    ../../modules/nixos/claudecode.nix
     ../../modules/nixos/docker.nix
     ../../modules/nixos/gnupg.nix
     ../../modules/nixos/locales.nix
@@ -26,9 +25,6 @@
   # host-specific niri config (input devices, laptop binds)
   home-manager.users.aaron.imports = [ ./niri.nix ];
 
-  # set hostname
   networking.hostName = "neon";
-
-  # install state version
-  system.stateVersion = "25.11"; # Don't change
+  system.stateVersion = "25.11"; # don't change
 }