Add comprehensive AGENTS.md documentation file

[Copilot]

This PR adds a comprehensive AGENTS.md file to the repository root that documents the project structure, architecture, coding conventions, testing protocols, and development workflow for the Azure AI Agents JavaScript demo.

What's Added

The AGENTS.md file provides detailed documentation covering:

• Project Structure: Complete directory layout with descriptions of each component
• Architecture Overview: Configuration-driven agent creation patterns and tool factory implementation
• Coding Conventions: TypeScript/ES module standards, naming conventions, and Azure SDK integration patterns
• Testing Protocols: Build validation, example execution, and interactive demo testing procedures
• Development Workflow: Setup instructions, development cycle, and guidelines for adding new agent types
• Pull Request Guidelines: Code quality requirements, PR description format, and review checklist

Implementation Details

The documentation is specifically tailored to this Azure AI Agents project and includes:

• Service-oriented architecture with clear separation between agents, tools, and threads
• Configuration-driven approach using PromptConfig objects for different agent types
• Tool factory pattern supporting code-interpreter, AI Search, function tools, and Bing Grounding
• Resource cleanup patterns to ensure proper disposal of agents and uploaded files
• Azure-specific integration requirements and authentication patterns

Why This Matters

This documentation will help contributors understand:

• How to properly structure code within the existing architecture
• Required coding standards and conventions for consistency
• Testing procedures to validate changes work correctly
• Development workflow for adding new features safely

The existing .github/copilot-instructions.md file was preserved unchanged as required by the issue specifications.

Fixes #7.

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