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>

autoinstall/user-data

@@ -1,7 +1,6 @@
 #cloud-config
 autoinstall:
   version: 1
-  interactive-sections: []
 
   # Locale, keyboard, timezone
   locale: en_US.UTF-8
@@ -105,6 +104,8 @@ autoinstall:
 
     # Disable cloud-init on the installed system (no longer needed after autoinstall)
     - curtin in-target --target=/target -- touch /etc/cloud/cloud-init.disabled
+    - curtin in-target --target=/target -- dpkg --configure -a
+    - curtin in-target --target=/target -- apt-get remove -y cloud-init cloud-guest-utils
 
   user-data:
     disable_root: false