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/ubuntu2604/user-data

@@ -0,0 +1,46 @@
+#cloud-config
+preserve_hostname: false
+manage_etc_hosts: true
+ssh_pwauth: true
+
+users:
+  - name: guofang
+    gecos: Guofang
+    groups: [sudo]
+    shell: /bin/bash
+    sudo: "ALL=(ALL) NOPASSWD: ALL"
+    lock_passwd: false
+    ssh_authorized_keys:
+      - @SSH_PUBKEY@
+    hashed_passwd: $6$0NtCERV3eO12t9oJ$nlFE2sSc66nFfuD5X0xMnOla1cuzOnHfeGNRY5X7dCDHg.WzWVmuRe7M17Puj44.eCWLnJAWt4WNjd9vP9X0D0
+
+chpasswd:
+  expire: false
+
+# amd64 packages are in the ubuntu archive, arm64 packages in ubuntu-ports.
+apt:
+  primary:
+    - arches: [amd64, i386]
+      uri: "https://mirrors.ustc.edu.cn/ubuntu"
+    - arches: [default]
+      uri: "https://mirrors.ustc.edu.cn/ubuntu-ports"
+  security:
+    - arches: [amd64, i386]
+      uri: "https://mirrors.ustc.edu.cn/ubuntu"
+    - arches: [default]
+      uri: "https://mirrors.ustc.edu.cn/ubuntu-ports"
+
+package_update: true
+
+packages:
+  - ubuntu-desktop-minimal
+  - ibus-table-wubi
+
+runcmd:
+  - systemctl set-default graphical.target
+
+power_state:
+  delay: now
+  mode: reboot
+  message: "GNOME installation completed; rebooting into graphical mode."
+  condition: true