docs: add code review guidelines and prompts for RIF Relay Contracts

[Hakob23]

What

• Add .github/copilot-instructions.md to guide GitHub Copilot during code reviews.
• Add .github/prompts/review-code.prompt.md with review areas and a structured feedback format.
• Align with the pattern used in rif-relay-server, rif-relay-client, and sibling RSK repos, with contracts-specific checks (verifiers, EIP-712, fund safety, proxy/factory integrity, Hardhat testing).

Why

• The repo had no Copilot or AI review guidance, while sibling RSK repos already use this pattern.
• Contract changes define on-chain invariants consumed by client and server — incorrect verifier or EIP-712 changes can cause fund loss or broken signatures across the ecosystem.
• Standardizes review expectations for both human reviewers and Copilot on PRs.

Refs

• https://github.com/rsksmart/rif-relay-server/blob/main/.github/copilot-instructions.md
• https://github.com/rsksmart/rif-relay-client
• https://github.com/rsksmart/rif-relay-server
• https://github.com/rsksmart/liquidity-provider-server/blob/master/.github/copilot-instructions.md

Jira Ticket

https://rsklabs.atlassian.net/browse/FLY-2377