chore(aquamarine): use NixOS's NPU module with nixos-unstable-small (#527)

* chore(aquamarine): take the NPU stack from nixos-unstable-small
* refactor(aquamarine): load the NPU driver via hardware.cpu.intel.npu

Assisted-by: DeepSeek Harness (claude-opus-5-5)

outputs/default.nix

@@ -61,7 +61,7 @@ let
         # to use chrome, we need to allow the installation of non-free software
         config.allowUnfree = true;
       };
-      pkgs-npu = import inputs.nixpkgs-npu {
+      pkgs-small = import inputs.nixpkgs-small {
         inherit system;
         config.allowUnfree = true;
       };