fix: enable flakes and nix-command and set NIX_PATH for nix-shell

core/system/nix.nix

@@ -0,0 +1,26 @@
+{ inputs, lib, ... }:
+
+{
+  nix = {
+    settings = {
+      # Enable experimental features for modern Nix commands and flakes
+      experimental-features = [ "nix-command" "flakes" ];
+      
+      # Allow users in the wheel group to manage the Nix store
+      trusted-users = [ "root" "@wheel" ];
+      
+      # Optimize storage by hard-linking identical files
+      auto-optimise-store = lib.mkDefault true;
+    };
+
+    # Populates NIX_PATH with the nixpkgs input from the flake.
+    # This is critical for making 'nix-shell -p ...' and other legacy
+    # Nix commands work in a flake-based environment without channels.
+    nixPath = [ "nixpkgs=${inputs.nixpkgs}" ];
+
+    # Map all flake inputs to the system registry.
+    # This ensures 'nix shell nixpkgs#...' uses the exact same version
+    # as the rest of the system and doesn't need to re-download.
+    registry = lib.mapAttrs (_: value: { flake = value; }) inputs;
+  };
+}