Add copilot instructions (#8676)

Author: dibarbet

.github/copilot-instructions.md

@@ -0,0 +1,62 @@
+# Copilot Coding Agent Instructions for `vscode-csharp`
+
+## Project Overview
+- This is the official C# extension for Visual Studio Code, supporting C# development via OmniSharp and Roslyn-based language servers.
+- The codebase is TypeScript/JavaScript, with some JSON and configuration files. It integrates with .NET components and external language servers.
+
+## Architecture & Key Components
+- **src/**: Main extension source. Key subfolders:
+  - `lsptoolshost/`: Hosts LSP (Language Server Protocol) logic, including Copilot integration (`copilot/`), Roslyn, and Razor support.
+  - `omnisharp/`: Protocols and logic for OmniSharp-based language server.
+  - `razor/`: Razor language support and configuration.
+- **Copilot Integration**:
+  - `src/lsptoolshost/copilot/contextProviders.ts` and `relatedFilesProvider.ts` register C# context and related files providers for GitHub Copilot and Copilot Chat extensions.
+  - The Roslyn Copilot language server is managed as a downloadable component (see `package.json` and `CONTRIBUTING.md`).
+
+## Developer Workflows
+- **Build**: `npm run compile` (or use VS Code build task)
+- **Test**: `npm test` (runs Jest tests)
+- **Package**: `npm run package` (creates VSIX)
+- **Dependencies**: Use `gulp installDependencies` to fetch .NET/LS components
+- **Debugging**: See `docs/debugger/` for advanced .NET debugging, including runtime and external library debugging.
+- **Roslyn Copilot Language Server**: To update/test, see instructions in `CONTRIBUTING.md` (triggers pipeline, checks logs for install, etc.)
+
+## Infrastructure Tasks
+- **Tasks Directory**: Build automation is in `tasks/` using Gulp. Key modules:
+  - `testTasks.ts`: Test orchestration for unit/integration tests across components
+  - `offlinePackagingTasks.ts`: VSIX packaging for different platforms (`vsix:release:package:*`)
+  - `componentUpdateTasks.ts`: Automated updates for Roslyn Copilot components
+  - `snapTasks.ts`: Version bumping and changelog management for releases
+  - `gitTasks.ts`: Git operations for automated PR creation and branch management
+- **Adding New Tasks**: Create `.ts` file in `tasks/`, define `gulp.task()` functions, require in `gulpfile.ts`
+- **Task Patterns**: Use `projectPaths.ts` for consistent path references, follow existing naming conventions (`test:integration:*`, `vsix:*`, etc.)
+
+## Project Conventions & Patterns
+- **TypeScript**: Follows strict linting (`.eslintrc.js`), including header/license blocks and camelCase filenames (except for interfaces and special files).
+- **Component Downloads**: Language servers and debuggers are downloaded at runtime; see `package.json` for URLs and install logic.
+- **Copilot Providers**: Use `registerCopilotContextProviders` and `registerCopilotRelatedFilesProvider` to extend Copilot context for C#.
+- **Testing**: Prefer integration tests over unit tests for features. Structure follows:
+  - `test/lsptoolshost/integrationTests/` for Roslyn/LSP features
+  - `test/omnisharp/omnisharpIntegrationTests/` for OmniSharp features  
+  - `test/razor/razorIntegrationTests/` for Razor features
+  - Use `test/*/integrationTests/integrationHelpers.ts` for test setup utilities
+  - Tests use Jest with VS Code test environment and require workspace test assets
+  - Run with `npm run test:integration:*` commands (e.g., `npm run test:integration:csharp`)
+
+## Integration Points
+- **GitHub Copilot**: Extension registers C# context and related files providers if Copilot/Copilot Chat extensions are present.
+- **Roslyn Language Server**: Managed as a downloadable component, versioned in `package.json` and updated via pipeline.
+- **OmniSharp**: Legacy support, also downloaded as a component.
+
+## Examples
+- To add a new Copilot context provider: see `src/lsptoolshost/copilot/contextProviders.ts`.
+- To update Roslyn Copilot server: follow `CONTRIBUTING.md` > "Updating the Roslyn Copilot Language Server version".
+
+## References
+- [README.md](../README.md): User-facing overview and features
+- [CONTRIBUTING.md](../CONTRIBUTING.md): Dev setup, packaging, and advanced workflows
+- [package.json](../package.json): Component download logic, scripts
+- [src/lsptoolshost/copilot/](../src/lsptoolshost/copilot/): Copilot integration logic
+
+---
+For any unclear or incomplete sections, please provide feedback to improve these instructions.