List entity nodes admitted by the current scope, optionally 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. Entities are scoped through connected memory nodes by the scope provider, honoring USER and TENANT isolation. Empty prefix returns entities by name.