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>

modules/nixos/settings.nix

@@ -1,31 +1,25 @@
 {
   nix = {
-    # nix settings
     settings = {
-      # enable flakes and nix-command
       experimental-features = [
         "nix-command"
         "flakes"
       ];
-      # auto-optimize my nix-store
       auto-optimise-store = true;
-      # use all cores
+      # parallel builds, each using all cores
       max-jobs = "auto";
-      # use all available cores per job
       cores = 0;
-      # add trusted substituters (binary caches)
-      substituters = [
-        "https://cache.nixos.org"
-        "https://nix-community.cachix.org"
-      ];
-      # add keys
-      trusted-public-keys = [
-        "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+      # cache.nixos.org is the default, these come on top
+      extra-substituters = [ "https://nix-community.cachix.org" ];
+      extra-trusted-public-keys = [
         "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
       ];
+      trusted-users = [
+        "root"
+        "@wheel"
+      ];
     };
 
-    # enable automatic garbage collection
     gc = {
       automatic = true;
       dates = "weekly";
@@ -33,20 +27,9 @@
     };
   };
 
-  # add trusted users (able to modify the nix store)
-  nix.settings.trusted-users = [
-    "root"
-    "@wheel"
-  ];
-
-  # allow unfree packages (steam, protonvpn, discord, etc.)
+  # steam, protonvpn, discord, etc.
   nixpkgs.config.allowUnfree = true;
 
   # links /libexec from derivations to /run/current-system/sw
   environment.pathsToLink = [ "/libexec" ];
-
-  # enable home-manager globally
-  home-manager.useGlobalPkgs = true;
-  home-manager.useUserPackages = true;
-  home-manager.backupCommand = "mv -f {file} {file}.hm-bak-$(date +%Y%m%d%H%M%S)";
 }