Add Ubuntu 26.04 virtual machine

Add run-ubuntu2604.py with its QEMU config files and NoCloud seed. The
launcher follows the Debian 13 one, with UBUNTU_VM_ settings and SSH
forwarded to host port 2225. The seed injects the current user's SSH public
key, uses the USTC mirror (ubuntu-ports on arm64) and installs the GNOME
desktop on first boot. Update vm-images for the ubuntu2604 image target.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kwxp2XWvKnuEBifnCbhyX5

seediso/README.md

@@ -7,7 +7,8 @@ Each operating system has an independent NoCloud configuration and generated
 seediso/
 ├── debian13/
 ├── almalinux10/
-└── fedora44/
+├── fedora44/
+└── ubuntu2604/
 ```
 
 Keeping per-OS metadata separate allows each VM to use a distinct hostname and