Add GitHub Copilot code review instructions

[feliperodri]

Add .github/copilot-instructions.md to guide GitHub Copilot's automated code reviews for this repository. This will help us triage the coming wave of challenge solutions.

The instructions cover review guidelines for the four types of PRs we receive:

• Challenge solutions — formal verification rigor (no vacuous proofs, justified assumptions, contracts matching safety docs), Rust std library code quality standards, and success criteria compliance.
• New challenge proposals — template and tracking issue requirements.
• New tool applications — CI workflow and book entry requirements.
• Maintenance — ensuring existing proofs are not weakened.

Also includes a list of common red flags to watch for (e.g., over-constrained assumptions, missing cfg gates, unapproved tools).

Reference: Copilot code review custom instructions.

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