Fix public Market Map proof verification URL

[elianguitarra]

/claim #1

Payout if accepted: Base USDC 0x237664403f52A15142795f807ADB3524E8057909.

Summary

This fixes the public proof readback issue from my no-spend QA pass on issue #1:

• publishes products/agent-commerce-market-map/verification.json, matching the proof.verificationUrl already advertised by /api/agent-products and /api/payment-request
• removes that public verification receipt from .gitignore while keeping private paid fulfillment assets ignored
• adds a lightweight regression script that checks catalog products with a sha256 proof have a local public verificationUrl asset whose productId and sha256 match the catalog

The paid packet remains private; the new JSON is only a public verification receipt for agents/buyers.

Verification

• node scripts/test-public-proof-urls.js -> public proof URL checks passed
• node scripts/verify-private-assets.js -> private fulfillment assets remain untracked/ignored and absent from catalog paths

Note: I also attempted node scripts/test-x402-receipt-verifier.js, but this fresh clone has no installed ethers dependency in node_modules, so that broader existing script could not run locally before npm install.