Disko 1.13.0+ rejects "100%FREE" in favor of "100%". Earlier
commit 139e5f3 (from agent 'isogonalconguate') had this fix but
got dropped during the agent-garbage cleanup (commit 430252a is
GRUB switch, 5e1b369 is systemd-boot, 99b887e is original disko).
This brings the fix back, with cleaned-up comments that reflect
current reality (GRUB bootloader, BIOS mode).
Session: verify plan
