mkarchiso reuses completed stages in the work dir -> a 'too short' build that
ships a stale airootfs. bootstrap now 'rm -rf' the work dir first. Also switch
profiledef bootmodes to the non-deprecated names (bios.syslinux, uefi.systemd-boot).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>