Export ML Notebook

Source of truth stays the # %% .py. This skill only writes derived .ipynb (and optional .nb.html). Do not rewrite the .py from the notebook. cells run is not a substitute.

Human-facing prose

Details: setup-workspace references/human_facing_prose.md. Tell the user the written .ipynb (and HTML viewer) paths. Do not quote notebook convert, --html, or site build as something they should run.