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/argon/configuration.nix

@@ -1,15 +1,25 @@
+# argon: desktop with an amd gpu and three luks-encrypted data drives.
+# The drives are unlocked in the initrd with keyfiles from /etc/nixos/keys.
+
+{ lib, pkgs, ... }:
+
+let
+  # luks name -> partition uuid, mounted at /mnt/<name>
+  drives = {
+    data1 = "dfae62cc-bad1-4879-bf9a-461bde833625";
+    data2 = "8312edae-9247-481b-a313-52a7f848f027";
+    nvmecache = "2352250e-4ebe-4f9a-bf66-0d4aaa961bd8";
+  };
+  key = name: "/etc/nixos/keys/${name}.key";
+in
 {
   imports = [
     ../../modules/nixos/appimage.nix
     ../../modules/nixos/astro.nix
     ../../modules/nixos/audio.nix
     ../../modules/nixos/bootloader.nix
-    ../../modules/nixos/calibre.nix
     ../../modules/nixos/certificates.nix
-    ../../modules/nixos/claudecode.nix
-    ../../modules/nixos/drives.nix
     ../../modules/nixos/gnupg.nix
-    ../../modules/nixos/graphics.nix
     ../../modules/nixos/greetd.nix
     ../../modules/nixos/locales.nix
     ../../modules/nixos/networking.nix
@@ -29,9 +39,33 @@
   # host-specific niri config (outputs, input devices)
   home-manager.users.aaron.imports = [ ./niri.nix ];
 
-  # set hostname
-  networking.hostName = "argon";
+  # amd gpu: mesa/vulkan, and amdgpu in the initrd for plymouth
+  hardware.graphics = {
+    enable = true;
+    enable32Bit = true;
+  };
+  hardware.amdgpu.initrd.enable = true;
 
-  # install state version
-  system.stateVersion = "25.11"; # Don't change
+  # encrypted data drives
+  boot.initrd.secrets = lib.mapAttrs' (name: _: lib.nameValuePair (key name) (key name)) drives;
+  boot.initrd.luks.devices = lib.mapAttrs (name: uuid: {
+    device = "/dev/disk/by-uuid/${uuid}";
+    keyFile = key name;
+  }) drives;
+  fileSystems = lib.mapAttrs' (
+    name: _:
+    lib.nameValuePair "/mnt/${name}" {
+      device = "/dev/mapper/${name}";
+      fsType = "ext4";
+      options = [ "nofail" ];
+    }
+  ) drives;
+
+  environment.systemPackages = with pkgs; [
+    amdgpu_top
+    calibre # ebook management and kobo syncing
+  ];
+
+  networking.hostName = "argon";
+  system.stateVersion = "25.11"; # don't change
 }

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
 }