Add GitHub Copilot instructions for AL development workflow

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to onboard GitHub Copilot coding agents to the ALAppExtensions repository. The instructions provide essential information to help coding agents work efficiently without extensive exploration and reduce the likelihood of build failures or rejected PRs.

What's included:

Repository Overview:

• Clear description of the AL app extensions for Business Central
• Repository statistics (~13,650 files, ~10,686 AL files)
• Technology stack (AL language, PowerShell, AL-Go framework)

Build & Validation Process:

• Step-by-step build commands using AL-Go framework
• Docker requirements for Business Central containers
• Local development environment setup
• Testing procedures and validation pipelines
• Critical timing information (15-30+ minute initial builds)

Project Architecture:

• Directory structure with Apps/, Build/, .github/ organization
• Configuration file locations and purposes
• App structure patterns and dependencies
• Key workflow files and validation processes

Development Guidelines:

• Contribution requirements (GitHub issue linking mandatory)
• Code standards and ruleset compliance
• Common pitfalls and how to avoid them
• AL-specific development patterns

Key benefits for coding agents:

1. Faster onboarding - No need to search for build instructions or project structure
2. Reduced failures - Clear warnings about Docker requirements and build processes
3. Compliance guidance - Explicit requirements for PR linking and code standards
4. Time savings - Direct paths to key files and configurations

The instructions are concise (under 2 pages) and focus on actionable information that will prevent common mistakes like attempting manual AL compilation or creating PRs without linked issues.

This addresses the repository onboarding goal by providing a single source of truth for coding agents to understand the AL development workflow in the ALAppExtensions repository.

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