Fetch a stored model's full Petri-net JSON by its content id. A name in place of an id is refused saying to search with sim_list_models' query, and that a model created without sign-in is listed for nobody, so its id is the one its create returned; or naming the curated model's id when a curated model has exactly that name. An id of another stored kind (a prompt or artifact, a sim_reach question or answer, a system, binding, map, collection, witness, relation, or sim_query query or result) is refused naming what it is and the tool or route that reads it. Public and free.