feat: Add comprehensive Copilot coding guidelines and code review

[rusty1968]

• Add detailed Copilot instructions with safety-first coding patterns
• Define test code exceptions for ergonomic development while maintaining safety
• Establish type-safe hardware register access requirements
• Add comprehensive code review checklist and forbidden patterns reference
• Create initial code review identifying 50+ violations in existing codebase

Key guidelines:

• Panic-free production code (no unwrap/expect/indexing)
• Type-safe PAC-based hardware register access only
• Named constants instead of magic numbers
• Test code may use unwrap/indexing for ergonomics but not bypass hardware safety
• Security-focused patterns for cryptographic operations

The code review identifies critical violations in ECDSA/RSA modules using direct register access and systematic unsafe patterns throughout the codebase requiring immediate refactoring.