fix(worktree): name the ambiguous path form instead of inferring it

The previous guard asked whether `join` had dropped its base, which a
drive-relative PROJECT path answers wrongly: `C:repo` joined with
`C:repo\..\victim` still starts with `C:repo`, so the check passed a path
whose meaning the process and the git child would each finish from their
own per-drive current directory.

Ask the question that was always meant instead — is either side relative
to a drive rather than rooted on one — and refuse whichever side carries
it. Unix has no prefixes, so the predicate is false for every path there;
what a Unix run pins is that no shape a real folder row holds is mistaken
for the Windows form.