Add comprehensive AI agent documentation for Musoq architecture and development

[Copilot]

This PR adds comprehensive documentation specifically designed for AI agents working with the Musoq codebase. The documentation provides deep technical insights into the SQL query engine's architecture, development patterns, and extension mechanisms.

What's Added

📚 Complete Documentation Suite in .copilot/

• Main Guide (README.md) - 14.5KB comprehensive overview covering architecture, components, development workflow, and testing strategies
• Architecture Deep Dive - Detailed technical analysis of Parser, Schema, Converter, Evaluator, and Plugin systems with internal implementation details
• Plugin Development Guide - Step-by-step reference for creating data sources and function libraries with real-world examples
• Development & Debugging Guide - Environment setup, debugging techniques, performance optimization, and troubleshooting
• API Usage Examples - Practical integration patterns with code examples for various scenarios
• Navigation Index - Quick reference and cross-linking between documentation sections

Key Documentation Features

Architecture Understanding

• Component interaction diagrams and data flow visualization
• AST processing pipeline from SQL text to executable C# code
• Plugin system architecture with schema contracts and method resolution
• Build chain pattern implementation for code generation

Development Workflow

• Environment setup for .NET 8.0 with proper IDE configuration
• Component-specific development cycles with testing strategies
• Build and CI/CD configurations with performance benchmarking
• Error handling patterns and debugging techniques

Plugin Development

• Complete data source plugin creation with ISchema implementation
• Advanced patterns like chunked processing, dynamic schemas, and parameterization
• Function library development with aggregations and complex operations
• Registration patterns and deployment strategies

Practical Examples

• In-memory, REST API, and database data source implementations
• Query parameterization and dynamic schema registration
• Streaming large datasets with cancellation token support
• Error handling with timeout management and logging integration

Technical Details

The documentation covers Musoq's unique architecture:

SQL Query → Lexer → Parser → AST → Converter → C# Code → Compiler → Assembly → Executor → Results

Key patterns include:

• Plugin Contract: ISchema → SchemaBase → Custom implementation
• Data Iteration: RowSource with lazy evaluation and cancellation support
• Code Generation: Build chain pattern with AST transformation stages
• Testing: BasicEntityTestBase with mock data and integration patterns

Why This Matters

Musoq transforms SQL queries into dynamically compiled C# code to query diverse data sources. This documentation enables AI agents to:

• Understand the sophisticated compilation pipeline
• Develop new data source plugins efficiently
• Debug complex query execution issues
• Optimize performance and memory usage
• Extend functionality with custom functions

The documentation is structured for both newcomers learning the architecture and experienced developers implementing advanced features.

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