Add GitHub Copilot instructions for repository conventions

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for nuget-license
+
+## Project Overview
+
+This repository contains **nuget-license**, a .NET tool for analyzing, printing, and validating licenses of NuGet package dependencies in .NET and C++ projects. The project supports .NET Core, .NET Standard, .NET Framework, and native C++ projects.
+
+## Project Structure
+
+- **src/**: Source code for the main tools
+  - **NuGetLicenseCore/**: Cross-platform .NET Core global tool
+  - **NuGetLicenseFramework/**: .NET Framework standalone executable
+  - **NuGetUtility/**: Core utility library
+  - **NuGetLicense/**: License validation and processing library
+  - **FileLicenseMatcher/**: License file matching library
+- **tests/**: Test projects
+  - **NuGetUtility.Test/**: Core utility tests
+  - **NuGetLicense.Test/**: License validation tests
+  - **FileLicenseMatcher.Test/**: File license matcher tests
+  - **NuGetUtility.UrlToLicenseMapping.Test/**: URL to license mapping tests
+  - **targets/**: Test target projects with various configurations
+- **integration/**: Integration test projects
+- **docs/**: Documentation files
+- **.github/workflows/**: CI/CD workflows
+
+## Building the Project
+
+### Prerequisites
+
+- .NET SDK 10.0.101 or later (with rollForward to support minor versions)
+  - The project also targets net8.0 and net9.0, which require their respective SDKs
+- MSBuild (for .NET Framework and C++ projects on Windows)
+
+### Build Commands
+
+```bash
+# Restore dependencies
+dotnet restore
+
+# Build the entire solution
+dotnet build --configuration Release --no-restore
+
+# Build a specific project
+dotnet build ./src/NuGetLicenseCore/NuGetLicenseCore.csproj --configuration Release
+
+# On Windows with MSBuild
+msbuild -t:rebuild -restore -p:RestorePackagesConfig=true -property:Configuration=Release
+```
+
+## Testing the Project
+
+### Running Tests
+
+```bash
+# Run all tests
+dotnet test --configuration Release --no-restore
+
+# Run specific test project
+dotnet test --project ./tests/NuGetUtility.Test/NuGetUtility.Test.csproj --configuration Release
+
+# Run tests for a specific framework
+dotnet test --project ./tests/NuGetUtility.Test/NuGetUtility.Test.csproj --configuration Release -f net10.0
+```
+
+### Test Organization
+
+- Tests follow the pattern `ClassName.Test.cs` for testing `ClassName.cs`
+- Use xUnit as the testing framework
+- Test projects include both unit tests and integration tests
+
+## Code Formatting and Linting
+
+```bash
+# Check code formatting (does not modify files)
+dotnet format --verify-no-changes
+
+# Format code
+dotnet format
+```
+
+## Coding Conventions
+
+### C# Code Style
+
+- **Indentation**: 4 spaces (never tabs)
+- **Line endings**: CRLF for most files, LF for shell scripts
+- **File header**: All C# files must include the license header:
+  ```csharp
+  // Licensed to the projects contributors.
+  // The license conditions are provided in the LICENSE file located in the project root
+  ```
+- **Namespaces**: Use block-scoped namespaces (not file-scoped)
+- **Braces**: Opening braces on new lines (Allman style)
+- **Using directives**: Outside namespace, sorted with System directives first
+
+### Naming Conventions
+
+- **Private/internal fields**: `_camelCase` with underscore prefix
+- **Static private/internal fields**: `s_camelCase` with `s_` prefix (preferred for new code)
+  - Note: Some ported code (e.g., FileLicenseMatcher from Java SPDX library) uses `UPPER_CASE` for static readonly fields
+- **Public static readonly fields**: `UPPER_CASE` (especially for constants and patterns in ported code)
+- **Constants**: `PascalCase`
+- **Public members**: `PascalCase`
+- **Local variables**: `camelCase`
+
+### Code Quality
+
+- Avoid `this.` qualifier for members (except when necessary for disambiguation)
+- Use explicit types instead of `var` for built-in types
+- Use modern C# features: pattern matching, null propagation, coalesce expressions
+- Prefer switch expressions over switch statements
+- Use expression-bodied members where appropriate
+- Always use braces for control flow statements
+
+### XML Documentation
+
+- Public APIs should have XML documentation comments
+- Use `<summary>`, `<param>`, `<returns>`, `<exception>` tags as appropriate
+
+## Common Patterns in This Codebase
+
+- License validation uses expression parsing and evaluation
+- Package information is read from `project.assets.json` files generated by `dotnet restore`
+- The tool supports multiple output formats: Table, Markdown, JSON, and JsonPretty
+- Configuration files use JSON format for mappings, overrides, and filters
+
+## Dependencies
+
+- Avoid adding new NuGet dependencies unless absolutely necessary
+- When adding dependencies, ensure they work with all target frameworks (net8.0, net9.0, net10.0, net472)
+- Consider cross-platform compatibility when adding dependencies
+
+## Multi-targeting Considerations
+
+- The project targets multiple .NET versions including .NET Framework 4.7.2
+- Test code changes on all target frameworks when modifying core functionality
+- Be aware of API availability differences between .NET Framework and .NET Core/5+
+
+## GitHub Actions Workflows
+
+- `.github/workflows/action.yml`: Main CI workflow that runs tests, code formatting checks, and license validation
+- `.github/workflows/release.yml`: Release workflow for publishing packages
+- The CI validates against multiple frameworks: net8.0, net9.0, net10.0, and net472
+
+## Additional Notes
+
+- The project includes both managed (.NET) and native (C++) project support
+- Windows-specific paths are handled specially in some integration tests
+- The tool does NOT restore NuGet packages itself - it reads from existing `project.assets.json` files