Reads a saved diagram by id or key. The default outline is a header (id, title, revision, time, hash), the brief, recent versions, one element per line nested by container, and edges as 'id: from -> to'. view 'dsp' returns the scene JSON; 'mermaid' returns Mermaid to edit and send back to diagram_edit. revision or asOf reads an earlier version; knownRevision, only what changed since; elements, only those elements and their neighbors.