Add comprehensive GitHub Copilot instructions for LogExpert repository onboarding

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to onboard the LogExpert repository for GitHub Copilot coding agents, significantly reducing exploration time and providing essential guidance for effective development.

What's Added

Comprehensive Documentation (~10,000 characters)

• Repository Overview: Complete description of LogExpert as a Windows log tail and analysis application with feature breakdown
• Technology Stack Details: .NET 8.0-windows, Windows Forms, Nuke Build System, NUnit testing framework
• Build Instructions: Step-by-step setup, prerequisites, and command examples for both Nuke build system and direct .NET commands
• Project Architecture: Detailed directory structure, plugin system explanation, and component relationships

Critical Build Information

• Environment Requirements: Documents the requirement for .NET 9.0.301 SDK and Windows development environment
• Known Issues & Workarounds: Cross-platform build limitations, .NET version conflicts, and specific error resolutions
• Validation Steps: Clear testing and verification procedures for code changes

Development Guidance

• Common Tasks: Instructions for adding columnizers, modifying UI components, and managing dependencies
• Best Practices: Code style adherence, architectural patterns, and plugin development guidelines
• CI/CD Documentation: GitHub Actions and AppVeyor pipeline explanations

Key Benefits for Coding Agents

1. Reduces Exploration Time: Agents won't need to search for build commands, project structure, or configuration details
2. Prevents Common Errors: Documents Windows-only nature, .NET version requirements, and build system limitations
3. Provides Context: Explains plugin architecture, Windows Forms patterns, and legacy codebase considerations
4. Enables Quick Onboarding: Complete guidance from environment setup to validation procedures

Additional Improvements

• Updated .gitignore to exclude .nuke/ build artifacts directory
• Removed accidentally committed Nuke build system artifacts from version control

The instructions follow the requirement of being under 2 pages while providing comprehensive, non-task-specific guidance that will benefit any future development work on the LogExpert repository. This should significantly improve the quality and efficiency of Copilot-generated code changes by providing essential context about this Windows-specific .NET application.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.