refactor(modules): rename system/ to nixos/

Clearer naming to distinguish NixOS system modules from
Home Manager modules (nixos/ vs home/).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/nixos/default.nix

@@ -0,0 +1,25 @@
+{pkgs, ...}: {
+  imports = [
+    ./time.nix
+    ./nix.nix
+    ./backlight.nix
+    ./boot.nix
+    ./amd.nix
+    ./hardware.nix
+    ./home-manager.nix
+    ./services.nix
+    ./user.nix
+    ./version.nix
+    ./fonts.nix
+    ./programs.nix
+    ./security.nix
+    ./variables.nix
+    ./kernel.nix
+  ];
+  environment.systemPackages = with pkgs; [
+    # FIXME: must be on their own app
+    git
+    cloudflared
+    cachix
+  ];
+}