Pre-flight: store a policy decision for the agent wallet

Evaluates the payment against the wallet policy and stores the decision as an intent.

Keyban welcome offer. An agent declaring capabilities: ["sponsored-x402"] in the request may have an x402 payment paid by Keyban, when the organization offer covers it: only for the wallet that earned the offer (its first Base agent wallet), on the offer network, before expiry and for the whole price.

The response then carries sponsored: true: the envelope from is the Keyban sponsor wallet, not the agent wallet, and the agent must not sign it. It fetches the signature from POST /v1/policy/intents/:id/sponsored-signature once the intent is approved.

Without the capability, or when the offer does not cover the payment, sponsored is absent and the agent signs from its own wallet as usual.