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>

flake.nix

@@ -35,6 +35,9 @@
             ./hosts/${hostName}/configuration.nix
             home-manager.nixosModules.home-manager
             {
+              home-manager.useGlobalPkgs = true;
+              home-manager.useUserPackages = true;
+              home-manager.backupCommand = "mv -f {file} {file}.hm-bak-$(date +%Y%m%d%H%M%S)";
               home-manager.extraSpecialArgs = { inherit inputs; };
               home-manager.users.aaron.imports = [
                 nixvim.homeModules.nixvim