On Windows, an older install (or a git checkout that materialized the
repo's symlinked data/scripts as plain "pointer" files) leaves a regular
file at <skill>/data or <skill>/scripts. copyDataAndScripts then calls
mkdir on that path, which throws EEXIST; under `uipro init --ai all` the
per-platform error is swallowed, leaving e.g. codex with the stale
pointer files and no real directories.
Add ensureCleanDir(): before mkdir, lstat the target and remove it if it
is not already a directory. Existing real directories are preserved
(re-install is unaffected). Verified with a repro harness.
Closes #237