feat: add Shopify refund guardrail integration

[sureshchouksey8]

Issue

Closes #7

Summary

Adds a runnable Shopify refunds/create webhook guardrail that verifies Shopify HMAC signatures, calls APort for payments.refund.v1 decisions, approves allowed refunds, and tags denied refunds for manual review.

Acceptance Criteria

• Complete Shopify app/refund guardrail example
• Uses environment variables for Shopify/APort credentials
• Verifies webhook signatures before processing
• Sends refund context to APort policy verification
• Includes setup docs and tests

Testing

• npm install && npm test in examples/ecommerce/shopify-guardrail

Submitted by Suresh.

[sureshchouksey8]

This is ready for review. I kept the Shopify refund guardrail scoped to a runnable integration example, with configuration docs and a fail-closed verification path so it can be reviewed independently of the other APort submissions.