Universal Commerce Protocol: what a customer sets2

Let platforms buy from a customer through the Universal Commerce Protocol, over the same checkout sessions, pricing callback, charging and refunds as agentic checkout. The customer lists the platforms it accepts by the address of their profile and says what its profile publishes; Skerry makes and keeps its signing key. A preview, off by default: every call answers 409 capability_coming_soon until it is switched on for the platform, and completing a checkout also needs delegated payment tokens.