refactor: major refactor into proper reusable modules. No logical changes.

modules/config/nix.nix

@@ -0,0 +1,58 @@
+{
+  inputs,
+  pkgs,
+  stateVersion,
+  ...
+}: {
+  environment.etc."nixos/configuration.nix".source = pkgs.writeText "configuration.nix" ''
+    assert builtins.trace "This is a dummy config, use colmena!" false;
+    { }
+  '';
+
+  nix = {
+    settings = {
+      auto-optimise-store = true;
+      allowed-users = ["@wheel"];
+      trusted-users = ["root" "@wheel"];
+      substituters = [
+        "https://nix-config.cachix.org"
+        "https://nix-community.cachix.org"
+      ];
+      trusted-public-keys = [
+        "nix-config.cachix.org-1:Vd6raEuldeIZpttVQfrUbLvXJHzzzkS0pezXCVVjDG4="
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      ];
+      cores = 0;
+      max-jobs = "auto";
+    };
+    daemonCPUSchedPolicy = "batch";
+    daemonIOSchedPriority = 5;
+    distributedBuilds = true;
+    extraOptions = ''
+      builders-use-substitutes = true
+      experimental-features = nix-command flakes
+      flake-registry = /etc/nix/registry.json
+    '';
+    nixPath = ["nixpkgs=/run/current-system/nixpkgs"];
+    optimise.automatic = true;
+    gc = {
+      automatic = true;
+      dates = "monthly";
+      options = "--delete-older-than 90d";
+    };
+    # Define global flakes for this system
+    registry = {
+      nixpkgs.flake = inputs.nixpkgs;
+      p.flake = inputs.nixpkgs;
+      pkgs.flake = inputs.nixpkgs;
+      templates.flake = inputs.templates;
+    };
+  };
+
+  system = {
+    extraSystemBuilderCmds = ''
+      ln -sv ${pkgs.path} $out/nixpkgs
+    '';
+    inherit stateVersion;
+  };
+}