Buyer Agent

1. With the merchant running and PAYMENT_MODE=mock:

``bash python examples/buyer_agent.py --merchant http://127.0.0.1:8122 ``

1. Observe: unpaid GET → 402 → signed proof → 200.
2. Inspect settlement in the response:

- authorization_verified: true - payment_verified: false - settlement_mode: mock

1. For Base Sepolia: set PAYMENT_MODE=base_sepolia, send tUSDC, pass --tx-hash 0x….

Retrying the same proof must fail with PAYMENT_PROOF_ALREADY_USED.