docs: write docs/datasets.md

Datasets (`add dataset`, `dataset download`, `dataset publish-version`, `dataset remove-version`, `run ingest`) is referenced in #1577's README Datasets section but has no dedicated guide.

Should cover: dataset shapes (session list, ground-truth file, trace filter), version lifecycle, how datasets are consumed by `run batch-evaluation` and `run recommendation`.

Out of #1577.

Start with the README Datasets section referenced in #1577 and inspect the CLI entry points for add dataset, dataset download, dataset publish-version, dataset remove-version, run ingest, run batch-evaluation, and run recommendation. Write docs/datasets.md covering the named dataset shapes, version lifecycle, and how the evaluation and recommendation commands consume datasets; done means the README can link to a complete dedicated guide.

Written by the indexing model from the issue text.