Artifact-type instantiation now requires a title, and store-backed types get their own fill guidance

Starting a new Artifact from a published type now requires passing a title (what the user called it, or a short descriptive name) alongside type_url, in addition to the type's own instructions being returned. The tool description was also reworded to distinguish types filled through their own data store from ones filled with page/data files: for store-backed types, the model should call the store-write path or run_script rather than publish files.