feat: flake NixOS config for jupiter + VirtualBox test image

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

jupiter/vm.nix

@@ -0,0 +1,22 @@
+{ config, pkgs, lib, modulesPath, ... }:
+
+# VirtualBox test image. Reuses services.nix but adds console/SSH login
+# credentials so you can actually get into the VM. Disk + bootloader are
+# provided by the virtualbox-image module, so hardware-configuration.nix
+# is intentionally NOT imported here.
+{
+  imports = [
+    (modulesPath + "/virtualisation/virtualbox-image.nix")
+    ./services.nix
+  ];
+
+  # Allow password login for testing (real host is key-only).
+  services.openssh.settings.PasswordAuthentication = lib.mkForce true;
+
+  # Login: erik / test  (change or remove for anything but local testing).
+  users.users.erik.initialPassword = "test";
+  users.users.root.initialPassword = "test";
+
+  # Guest additions for clipboard/resize (optional).
+  virtualisation.virtualbox.guest.enable = true;
+}