install_phase.sh exited when it could not mark the backup `installing`,
before the installer ran, without starting the monitor phase 1 had stopped
(decision 2) and leaving the backup behind. Nothing was changed at that
point, so it now removes the backup, restarts the monitor if it ran,
releases the marker and exits 1.
SKILL.md's exit row said 1 for every failure; a failed registry conversion
after a good install passes the converter's nonzero code through, and a
cancelled prompt exits 0.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>