Fully unattended USB install: patch GRUB and remove cloud-init

- build-usb.sh now patches the ISO's grub.cfg with xorriso to add
  'autoinstall' kernel param (skips confirmation prompt) and reduces
  GRUB timeout from 30s to 5s
- Disable and remove cloud-init on the installed system to prevent
  boot delays on air-gapped network
- Fix ISO size calculation to use patched ISO for CIDATA partition offset

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build-usb.sh

@@ -105,14 +105,28 @@ done
 
 # --- Write ISO to USB ---
 echo "[2/6] Writing Ubuntu ISO to $USB_DEV (this may take several minutes)..."
-dd if="$ISO_PATH" of="$USB_DEV" bs=4M status=progress oflag=sync
+
+# Patch ISO to add 'autoinstall' kernel parameter (skips confirmation prompt)
+PATCHED_ISO=$(mktemp /tmp/ubuntu-autoinstall-XXXXXX.iso)
+cp "$ISO_PATH" "$PATCHED_ISO"
+PATCHED_GRUB=$(mktemp /tmp/grub-XXXXXX.cfg)
+xorriso -osirrox on -indev "$ISO_PATH" -extract /boot/grub/grub.cfg "$PATCHED_GRUB" 2>/dev/null
+sed -i 's|linux\t/casper/vmlinuz  ---|linux\t/casper/vmlinuz autoinstall ---|' "$PATCHED_GRUB"
+sed -i 's/^set timeout=30/set timeout=5/' "$PATCHED_GRUB"
+xorriso -indev "$PATCHED_ISO" -outdev "$PATCHED_ISO" \
+    -map "$PATCHED_GRUB" /boot/grub/grub.cfg \
+    -boot_image any replay 2>/dev/null
+echo "    Patched GRUB: added 'autoinstall' kernel param, reduced timeout to 5s"
+
+ISO_SIZE=$(stat -c%s "$PATCHED_ISO")
+dd if="$PATCHED_ISO" of="$USB_DEV" bs=4M status=progress oflag=sync
 sync
+rm -f "$PATCHED_ISO" "$PATCHED_GRUB"
 
 # --- Find the end of the ISO to create CIDATA partition ---
 echo "[3/6] Creating CIDATA partition after ISO data..."
 
 # Get ISO size in bytes and calculate the start sector for the new partition
-ISO_SIZE=$(stat -c%s "$ISO_PATH")
 SECTOR_SIZE=512
 # Start the CIDATA partition 1MB after the ISO ends (alignment)
 START_SECTOR=$(( (ISO_SIZE / SECTOR_SIZE) + 2048 ))