test: simplify hooks temp-dir cleanup to a one-line exit handler (#221)

#213 guarded cleanup with a flag + named function + process.once. But
fs.rmSync with force:true already no-ops on a missing path, so the guard
and the explicit end-of-file call are unnecessary. Collapse to a single
process.on('exit') handler.

Refs #204

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>