Set up GitHub Copilot instructions for repository

[Copilot]

This PR implements GitHub Copilot instructions to help coding agents understand and work effectively with the Mina Rust codebase.

What was added

Created .github/copilot-instructions.md with comprehensive guidance covering:

• Project overview - Mina protocol context and Redux-style state machine architecture
• Architecture patterns - State/Actions/Reducers flow, file organization conventions, and defensive programming practices
• Development workflow - Pre-change requirements, formatting procedures, and testing commands
• Code style guidelines - Rust conventions, commit message standards, and documentation formatting
• Common tasks - Feature development, debugging approaches, and dependency management

Key highlights

The instructions emphasize critical repository requirements:

• Mandatory trailing whitespace management (make fix-trailing-whitespace)
• Strict formatting standards for Rust, TOML, and markdown files
• Commit message rules (no AI co-authors, no emojis, 80-character wrapping)
• Understanding of the Redux architecture central to this blockchain implementation

Implementation approach

The content was carefully adapted from the existing comprehensive CLAUDE.md file, ensuring consistency with established development practices while being specifically tailored for GitHub Copilot agents.

All formatting checks pass and the file follows repository markdown standards.

Fixes #1423.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.