fix: Remove direct execution from unattended-upgrades.sh

- Remove configure_unattended_upgrades call at end of file
- Script is now library-only, called from setup.sh where DISTRO is set
- Prevents 'DISTRO: unbound variable' error when run standalone

lib/unattended-upgrades.sh

@@ -90,6 +90,3 @@ if ! command -v log_info &>/dev/null; then
     log_success() { echo "[SUCCESS] $1"; }
     log_error() { echo "[ERROR] $1" >&2; }
 fi
-
-# Main execution
-configure_unattended_upgrades