Default the image account to grenade

The account the image creates was fedora/fedora; make it grenade/grenade and
say in both docs that config/device.env decides it. Nothing else referenced the
name — sudo comes from wheel membership, not a per-user sudoers file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011XgGF5wfxLDAybVnNz6eNQ

README.md

@@ -26,8 +26,9 @@ Two variants:
 | `minimal`     | 8 GiB | Console, sshd, and enough tools to debug the machine |
 | `workstation` | 16 GiB| GNOME desktop                                |
 
-Default login is `fedora` / `fedora`, and the password must be changed at first
-login. Root is locked.
+Default login is `grenade` / `grenade`, and the password must be changed at
+first login. Root is locked. Both are set in `config/device.env`
+(`DEFAULT_USER`, `DEFAULT_PASSWORD`) if you want a different account.
 
 ## Hardware status