Adds copilot instructions and prompts

[Adam-it]

🎯 Aim

Add GitHub Copilot custom instructions and reusable prompts to improve AI-assisted development experience for the SharePoint Framework (SPFx) Toolkit VS Code Extension.

📷 Result

✅ What was done

• Added .github/copilot-instructions.md with comprehensive extension architecture documentation
• Created .github/prompts/create-pr-description.prompt.md for automated PR description generation
• Documented key architectural components (Extension Host, Chat Participant, Webview Panel, Service Layer, Authentication)
• Outlined development workflow including build system, NPM scripts, and VS Code tasks
• Provided project structure patterns for core services, chat integration, and UI components
• Documented critical dependencies and Microsoft 365 integration patterns
• Included development conventions for command registration, constants management, and error handling
• Added SPFx-specific knowledge including project detection, supported operations, and sample gallery integration

🔗 Related issue

internal suggestion

[nicodecleyre]

Nothing to add. Great work as always 🙏 You rock 🤩