list work_item:phase_artifacts renders table columns from the wrong noun (work_item fields, not artifacts)

`pkg/spec/autonomous_work.spec.yaml:287` has no `columns:` or `fields_noun`, so it defaults to `work_item`'s fields (`title`, `state`, etc.) instead of the artifact fields it actually returns. Table output shows wrong/blank columns.

Fix: add `columns:` or a `fields_noun` pointing at a noun matching the artifact shape.
