New artifact-type 'list instances' capability, end-to-end but with no visible in-slice trigger

A new "list instances" capability for Artifact types is now fully wired end-to-end: a client call to GET /types/{slug}/instances behind the artifact_type_instances feature flag, a tool-result builder returning fields like type, scope, instances, curated, hidden, more, overflow, dropped, and unavailable, and new terminal rendering showing either "instance listing not available to this account" or "listed N artifact(s) of this type". A related new "opened" artifact-link result renderer shows "Opened" with the resolved title/URL. Nothing yet in this area exposes the tool to a model, so what triggers this listing is not yet visible.