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/.gitignore

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+seed.iso
+*.iso

seediso/ubuntu2604/README.md

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+# Ubuntu 26.04 seed.iso
+
+This directory contains the cloud-init NoCloud configuration used by
+`run-ubuntu2604.py`.
+
+1. Edit `user-data`, `meta-data`, and (if needed) `network-config`.
+2. Build the image:
+
+   ```bash
+   ./seediso/ubuntu2604/build-seed.sh
+   ```
+
+   To choose another output path:
+
+   ```bash
+   ./seediso/ubuntu2604/build-seed.sh /path/to/seed.iso
+   ```
+
+The script uses `cloud-localds` when available, then falls back to `xorriso`,
+`genisoimage`, or `mkisofs`. Attach the resulting ISO as a CD-ROM when booting
+a cloud-init-enabled virtual machine.

seediso/ubuntu2604/build-seed.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+script_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+output="${1:-${script_dir}/seed.iso}"
+files=(user-data meta-data)
+
+# Public key injected into user-data: $SSH_PUBKEY_FILE, else the first
+# ~/.ssh/id_*.pub, else the first key offered by ssh-agent.
+pubkey=""
+if [[ -n "${SSH_PUBKEY_FILE:-}" ]]; then
+  pubkey="$(head -n1 -- "${SSH_PUBKEY_FILE}")"
+else
+  for f in "${HOME}"/.ssh/id_ed25519.pub "${HOME}"/.ssh/id_*.pub; do
+    if [[ -f "$f" ]]; then pubkey="$(head -n1 -- "$f")"; break; fi
+  done
+  if [[ -z "$pubkey" ]] && command -v ssh-add >/dev/null 2>&1; then
+    pubkey="$(ssh-add -L 2>/dev/null | head -n1 || true)"
+  fi
+fi
+if [[ "$pubkey" != ssh-* && "$pubkey" != ecdsa-* && "$pubkey" != sk-* ]]; then
+  printf 'Error: no SSH public key found; set SSH_PUBKEY_FILE.\n' >&2
+  exit 1
+fi
+
+work="$(mktemp -d)"
+trap 'rm -rf -- "${work}"' EXIT
+template="$(<"${script_dir}/user-data")"
+printf '%s\n' "${template//@SSH_PUBKEY@/${pubkey}}" > "${work}/user-data"
+cp -- "${script_dir}/meta-data" "${work}/meta-data"
+if [[ -f "${script_dir}/network-config" ]]; then
+  cp -- "${script_dir}/network-config" "${work}/network-config"
+fi
+
+mkdir -p "$(dirname -- "$output")"
+
+if [[ -f "${script_dir}/network-config" ]]; then
+  files+=(network-config)
+fi
+
+if command -v cloud-localds >/dev/null 2>&1; then
+  args=()
+  if [[ -f "${script_dir}/network-config" ]]; then
+    args+=(--network-config="${work}/network-config")
+  fi
+  cloud-localds "${args[@]}" "${output}" \
+    "${work}/user-data" "${work}/meta-data"
+elif command -v xorriso >/dev/null 2>&1; then
+  xorriso -as mkisofs -quiet -volid cidata -joliet -rock \
+    -output "${output}" "${files[@]/#/${work}/}"
+elif command -v genisoimage >/dev/null 2>&1; then
+  genisoimage -quiet -output "${output}" -volid cidata -joliet -rock \
+    "${files[@]/#/${work}/}"
+elif command -v mkisofs >/dev/null 2>&1; then
+  mkisofs -quiet -output "${output}" -volid cidata -joliet -rock \
+    "${files[@]/#/${work}/}"
+else
+  printf 'Error: install cloud-image-utils, xorriso, genisoimage, or mkisofs.\n' >&2
+  exit 1
+fi
+
+printf 'Created %s\n' "${output}"

seediso/ubuntu2604/meta-data

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+instance-id: ubuntu2604-host-001
+local-hostname: ubuntu2604-host

seediso/ubuntu2604/network-config

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+version: 2
+ethernets:
+  all-en:
+    match:
+      name: "en*"
+    dhcp4: true
+  all-eth:
+    match:
+      name: "eth*"
+    dhcp4: true

seediso/ubuntu2604/user-data

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+#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