Add comprehensive technical guide for VS Code Copilot chat session files This document provides complete technical documentation for understanding, discovering, and processing VS Code Copilot chat session files, based on empirical analysis and testing. Key content areas: - File location and discovery patterns across platforms - JSON structure and format specifications - Real-time session writing behavior analysis - Cross-platform implementation examples - Performance considerations for large files - Troubleshooting common issues - Complete code examples and interfaces This guide serves as the definitive reference for extension developers working with Copilot session data.