Add comprehensive GitHub Copilot instructions for fri_basic_rustling repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with exhaustive instructions for working effectively in the fri_basic_rustling codebase.

What's Included

The instructions provide validated, step-by-step guidance covering:

Build and Development Workflow:

• Complete bootstrap process with exact timing measurements (9-22 seconds for builds)
• Validated commands for development and release builds
• Module-specific testing commands with expected results
• Code quality tools (rustfmt, clippy) with current warning counts

Architecture Overview:

• Detailed descriptions of all 4 core modules (755 lines of Rust code total)
• Dependency documentation (ff, hex, rand, rs_merkle)
• Module interaction patterns and data flows
• Repository structure with file locations

Validation Scenarios:

• Complete FRI algorithm demonstration workflow
• Expected output patterns for commitment/decommitment phases
• Known issues documentation (failing symmetry test)
• Performance characteristics and timing expectations

Troubleshooting and Guidelines:

• Common failure modes and solutions
• Development best practices for finite field operations
• Expected test results (12 pass, 1 known failure)

Validation Process

All commands and timing measurements were exhaustively tested from a clean repository state:

# Bootstrap (validated timing)
cargo build          # 9-22 seconds - NEVER CANCEL
cargo build --release # 9 seconds - optimized build

# Testing (validated results)  
cargo test polynome      # 5 tests pass, < 1 second
cargo test fri_code_layer # 6 pass, 1 known failure, < 1 second  
cargo test channel       # 1 test passes, < 1 second

# Demo validation
cargo run  # Shows FRI commitment/decommitment working correctly

The instructions emphasize the educational nature of this FRI algorithm implementation while providing complete operational guidance for any developer working on the codebase for the first time.

Fixes #13.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.