Get an intent by id

Intent state. A completed payment intent's data carries paymentCredential (the credential to present to the seller) and transaction (the same shape send/transfer serve); data.x402 is a deprecated protocol-named copy of both. A payment carried by a movement plan carries movement: its nextStep reads wait while a person decides it, an admin signs it, or its signature is in flight, then done or blocked. A payment a person approved is signed by an admin, not the agent; an agent that restarted before signing a payment nobody had to approve sends its original request again on the same Idempotency-Key. status cannot carry this on its own, because a payment an admin is signing and one whose signature is in flight both read processing. A policy proposal carries proposal, whose reason says whether to wait or resubmit; once completed, its data names the policy approval created and the counterparty each proposed reference became.