Get report emergent signals

Returns structured emergent SIGINT signals inferred from a report. Reports created before signal extraction return an empty `signals` array. 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`.