Add comprehensive .github/copilot-instructions.md for Azure CLI development guidance

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to onboard GitHub Copilot coding agents to the Azure CLI repository. The instructions provide essential information to help agents work efficiently without extensive exploration.

What's Added

The instructions file covers:

Repository Overview

• Microsoft Azure CLI as a multi-platform command-line interface
• Python 3.9-3.12 codebase with 4 core modules and extensive tooling
• Clear guidance to use azdev (NOT the deprecated dev_setup.py)

Essential Development Setup

• Step-by-step setup process with proper command ordering
• Virtual environment requirements and activation
• Network timeout troubleshooting (common issue in this repository)
• Timing expectations for each setup step

Build and Validation Commands

• Style checking: azdev style (~30 seconds)
• Comprehensive linting: azdev linter --ci-exclusions --min-severity medium (2-3 minutes)
• Testing commands with module-specific and full test suite options
• Command coverage verification: azdev cmdcov {module}
• Package building procedures for various platforms

Project Architecture

• Core module structure in src/ directory
• Key directories and their purposes (scripts/, build_scripts/, doc/, etc.)
• Configuration files (.flake8, pylintrc, linter_exclusions.yml)
• CI/CD pipeline structure using Azure DevOps and GitHub Actions

Best Practices and Troubleshooting

• Testing patterns including serial vs parallel execution
• Code quality standards and coverage requirements
• Common issues and their solutions
• Important guidance for agents to minimize exploration time

Why This Matters

This file significantly improves the experience for coding agents by:

• Reducing the likelihood of PR rejections due to build failures or validation issues
• Minimizing bash command failures through validated procedures
• Providing clear guidance on repository structure and conventions
• Documenting timing expectations to prevent timeouts
• Including troubleshooting for common network and build issues

The instructions are based on comprehensive analysis of the repository's documentation, build pipelines, CI/CD configuration, and development workflows to ensure accuracy and completeness.

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