Add new variable CLEANUP_AUTO_PACKAGES that marks packages as
auto-installed instead of removing them directly. This allows
apt-get autoremove to safely remove packages that are not required
by user-selected groups during live installation.

Implementation uses preinstall hook to ensure correct ordering:
1. preinstall: mark CLEANUP_AUTO_PACKAGES as auto
2. pkg-install: mark .base and selected groups as manual
3. postinstall: autoremove removes only orphaned auto packages