demo(payments): enforce policy before signing

[EfeDurmaz16]

Summary

• Add a tiny local payment policy guard for the payments demo
• Enforce policy after payment request verification but before returning an execution URL or signing the receipt-service payload
• Cover approved, approval-required, and denied policy outcomes with focused tests
• Document that this is an example pattern, not a normative ACK-Pay policy engine

Fixes #91

Verification

• pnpm --filter ./demos/payments check:types
• pnpm --filter ./demos/payments exec vitest run
• pnpm exec oxfmt --check demos/payments/README.md demos/payments/src/payment-policy.ts demos/payments/src/payment-policy.test.ts demos/payments/src/payment-service.ts
• git diff --check

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📥 Commits

Reviewing files that changed from the base of the PR and between 6f65936 and f0f5df8.

📒 Files selected for processing (4)

• demos/payments/README.md
• demos/payments/src/payment-policy.test.ts
• demos/payments/src/payment-policy.ts
• demos/payments/src/payment-service.ts

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Do not trust each request's own issuer

When this payment service is reachable by any client, a third party can submit a valid PaymentRequest JWT it signed itself with a Stripe option whose recipient is that same DID; because trustedRequestIssuer is set from the token's own parsed.issuer, the empty allowlist is bypassed and the service will return a payment URL and later sign the receipt-service payload for an unapproved merchant. The policy should compare against a configured trusted server/merchant DID or explicit allowlist, not the self-asserted issuer from the request being evaluated.

Useful? React with 👍 / 👎.

[EfeDurmaz16]

Addressed in f0f5df8 by removing approval based on the Payment Request token’s own issuer. The demo now derives the autonomous recipient allowlist from the configured server identity and keeps self-asserted recipients in approval_required unless explicitly allowlisted.\n\nVerification:\n- pnpm --filter ./demos/payments test -- payment-policy.test.ts\n- pnpm exec oxfmt --check demos/payments/README.md demos/payments/src/payment-policy.ts demos/payments/src/payment-policy.test.ts demos/payments/src/payment-service.ts\n- git diff --check\n\nI also tried pnpm --filter ./demos/payments check:types, but this checkout currently fails to resolve the workspace agentcommercekit package and reports existing demo type errors outside this change.