Get report ontology graph

Returns the report's bounded ontology graph: report-local nodes, edges, incident anchors, citations, and counts. Reports created before graph extraction return `graph: null` with zero counts. Requires the `reports:read` scope. Supports x402 pay-per-request. Requests with a valid Bearer token use API-key authentication. Without a Bearer API key, start the x402 flow from the `402 Payment Required` response and `PAYMENT-REQUIRED` header; retry with `PAYMENT-SIGNATURE`.