The pre-check used resolve() before the Promise was created, so it
was undefined — the handler returned undefined and the script never
ran. Restructured so pre-check failures return directly from the
async handler, and only the script execution uses Promise/resolve.

All error paths now include full diagnostics: Python path, script
path, course dir existence, package import check results.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>