restructure folder organisation

legacy/modules/nix.nix

@@ -0,0 +1,41 @@
+{ config, lib, pkgs, inputs, ... }:
+{
+  nix = {
+    settings = {
+      trusted-users = [
+        "root"
+        "@wheel"
+      ];
+      auto-optimise-store = true;
+    };
+
+    gc = {
+      automatic = true;
+      options = "--delete-older-than 30d";
+    };
+
+    extraOptions = ''
+      experimental-features = nix-command flakes
+    '';
+
+    registry = {
+      nixpkgs.flake = inputs.nixpkgs;
+      unstable.flake = inputs.nixpkgs-unstable;
+    };
+  };
+
+  # support auto upgrade with flakes
+  system.autoUpgrade.flags = [
+    "--update-input"
+    "nixpkgs"
+    "--commit-lock-file"
+  ];
+
+  environment.systemPackages = with pkgs; [
+    nix-prefetch
+    nix-update
+    nixpkgs-fmt
+    nixpkgs-lint
+    nixpkgs-review
+  ];
+}