vm: add aarch64 UTM test target + cleanup pi/qmd placement

- nixosConfigurations.vm: minimal NixOS for aarch64 UTM testing
  - modules/vm.nix: network, SSH, user, no display manager
  - imports shared + wgu home configs, no desktop/Hyprland
- Moved pi/qmd from sscoble.nix → desktop.nix (clean separation)

home/desktop.nix

@@ -3,6 +3,9 @@
 {
   # === Linux desktop packages ===
   home.packages = with pkgs; [
+    # pi + qmd (from overlay)
+    pi-coding-agent
+    qmd
     # Terminal
     ghostty
     zellij

home/sscoble.nix

@@ -32,15 +32,10 @@
     curl
     wget
   ] ++ (if pkgs.stdenv.isLinux then (with pkgs; [
-    # Linux-specific packages
-    pi-coding-agent
-    qmd
     nodejs
     bun
     git
-  ]) else [
-    # macOS: pi, qmd, node, bun, git come from homebrew
-  ]);
+  ]) else []);
 
   # === Home directory (conditional) ===
   home.homeDirectory = if pkgs.stdenv.isDarwin