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

core/system/default.nix

@@ -4,6 +4,7 @@
   imports = [
     ./options.nix
     ./state.nix
+    ./nix.nix
     ./systemd.nix
     ./virtualization.nix
     ./fonts.nix

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;
+  };
+}

core/system/scripts/nomarchy-update

@@ -18,7 +18,7 @@ echo "--- Starting Nomarchy Update ---"
 
 # 1. Update Flake Lock
 echo "Updating flake inputs..."
-sudo nix --extra-experimental-features "nix-command flakes" flake update --flake "$REPO_DIR"
+sudo nix flake update --flake "$REPO_DIR"
 
 # 2. Rebuild System
 echo "Applying system-level updates..."

hosts/installer-iso.nix

@@ -5,7 +5,10 @@
 # This creates a minimal, text-only installation environment.
 # No desktop environment - just TTY with gum-based installer.
 
-{
+  imports = [
+    ../core/system/nix.nix
+  ];
+
   # Base installation media configuration is handled by the module imported in flake.nix
 
   # Console configuration for a pleasant TTY experience