Create idempotent invoices from pull requests

[phucnguyen1707]

Summary

• add a PR-only /coinpay create $10 USD --wallet <address> flow that derives evidence from the pull request and up to five linked closing issues
• generate deterministic payment identities so retries cannot create duplicate CoinPayPortal payments
• require maintainer authorization and an explicit destination wallet, while failing closed for contributor requests
• harden hidden-marker trust, author identity checks, untrusted text neutralization, amount validation, API failures, and retry behavior
• update the Action inputs, example workflow, documentation, committed bundle, and contract/e2e coverage

Validation

• Node.js 24.9.0
• pnpm run check
• 7 test files, 97 tests
• rebuilt dist/index.js twice and verified deterministic output
• checked the adapter against the current CoinPayPortal invoice/payment contract