Review feedback (#40998): `rm -rf` / `Remove-Item -Recurse -Force` on the
install dir is destructive -- a user might still want whatever is there.
Rename the broken checkout to a timestamped `<dir>.broken-<ts>` backup and
re-clone fresh, so nothing is ever deleted. Transient cleanup of a clone
attempt that fails within the same run is left as-is.
Behavioral coverage for install.sh's clone_repo() guard (removes a
commit-less checkout, keeps a real one, ignores a non-repo dir) plus a
contract check that install.ps1's repo-validity gate requires a resolvable
HEAD.