Set up comprehensive GitHub Copilot instructions file

[Copilot]

This PR adds a comprehensive .copilot-instructions.md file to provide GitHub Copilot with detailed context about the Cosmian KMS project structure, coding conventions, and key concepts.

The instructions file includes:

Project Overview

• Description of Cosmian KMS as a FIPS 140-3 compliant Key Management System
• Key features including KMIP protocol support, HSM integration, and post-quantum cryptography
• High-level architecture and purpose as both KMS and PKI

Technical Architecture

• Multi-crate workspace structure with clear separation between client and server components
• Detailed breakdown of core crates: server, kmip, crypto, access, server_database
• HSM integration modules for Proteccio, Utimaco, and SoftHSM2

Development Guidelines

• Error handling patterns using KResult<T> type alias and helper traits
• Configuration management supporting both CLI and TOML formats
• Async programming patterns with Tokio runtime
• Security best practices for cryptographic operations

Key Concepts

• KMIP operations and object lifecycle management
• HSM key prefixing scheme (hsm::<slot_number>::<key_identifier>)
• Authentication mechanisms (JWT/OIDC, certificate-based)
• Supported cryptographic algorithms and FIPS compliance

Code Examples

• Common patterns for configuration structs with clap derive macros
• Error handling with context and reason methods
• KMIP request processing patterns

This comprehensive context will help GitHub Copilot provide more accurate and project-specific code suggestions that align with the established patterns, security requirements, and architectural decisions of the Cosmian KMS codebase.

Fixes #501.

---

💡 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.

[Copilot]

@Manuthor 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.