List entity nodes in the current workspace, optionally filtered by name prefix. Backs Discord-style entity autocomplete (`/recall about:<entity>`). Entities are graph nodes (`node_category = 'entity'`), not metadata fields — calling `/by-metadata?metadata_key=entities` does NOT work because entities are edge-connected, not embedded in a metadata list. Workspace-scoped via `scope.workspace_id`. Empty prefix returns the most recent `limit` entities by name order.