Build service

Services are priced capabilities that agents can buy through x402.

await stellar.services.register({
  id: "research.summary.v1",
  name: "Research summary",
  price: { amount: "1.00", asset: "XLM" },
  sellerAgentId: "agent-cipher",
})

Expose metadata, price clearly, emit receipts, and use reputation events after successful delivery.