Add GitHub Copilot instructions for repository conventions and workflows

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for Teams.NET SDK
+
+This repository contains the Teams SDK for .NET, a suite of packages for building applications on the Teams Platform.
+
+## Build and Development Commands
+
+### Build
+```bash
+dotnet build
+```
+
+### Clean
+```bash
+dotnet clean
+```
+
+### Format Code
+```bash
+dotnet format
+```
+
+### Test
+```bash
+dotnet test
+```
+
+For verbose test output:
+```bash
+dotnet test -v d
+```
+
+### Restore Dependencies
+```bash
+dotnet restore
+```
+
+## Project Structure
+
+- **Libraries/** - Core SDK packages for Teams applications
+  - `Microsoft.Teams.Apps` - Core application framework
+  - `Microsoft.Teams.AI` - AI/ML capabilities for Teams
+  - `Microsoft.Teams.Api` - Teams API client
+  - `Microsoft.Teams.Cards` - Adaptive Cards support
+  - `Microsoft.Teams.Common` - Shared utilities and types
+  - `Microsoft.Teams.Extensions.*` - Extension packages for configuration, hosting, logging, and Graph
+  - `Microsoft.Teams.Plugins.*` - Plugin packages for ASP.NET Core, BotBuilder, and external integrations
+- **Samples/** - Example applications demonstrating SDK usage
+- **Tests/** - Unit and integration tests for all packages
+
+## C# Coding Conventions
+
+### General Style
+- Use **4 spaces** for indentation in C# files (not tabs)
+- Follow .NET naming conventions:
+  - PascalCase for types, methods, properties, public fields
+  - camelCase for local variables, parameters
+  - Private fields: `_camelCase` (underscore prefix)
+  - Private static fields: `s_camelCase`
+  - Interfaces: `IPascalCase` (I prefix)
+- Use **file-scoped namespaces** (preferred)
+- Place opening braces on new lines
+
+### Code Preferences
+- **Avoid `var`** - Use explicit types for built-in types and when type is not apparent
+- Use **expression-bodied members** for properties and accessors (when simple)
+- Prefer **pattern matching** over type checks and casts
+- Use **null-conditional operators** (`?.`) and **null-coalescing** (`??`)
+- Use **collection initializers** and **object initializers** when appropriate
+- Prefer **foreach** over traditional for loops when possible
+
+### Using Directives
+- Place using directives **outside namespace**
+- Sort system directives first
+- Separate import directive groups
+- **Unused using statements are errors** (IDE0005)
+
+### Documentation
+- Add XML documentation comments for public APIs
+- Include `<summary>`, `<param>`, and `<returns>` tags as appropriate
+
+## Testing Guidelines
+
+- Tests are located in the `Tests/` directory
+- Test project naming: `{LibraryName}.Tests`
+- Use xUnit for testing framework (if present in existing tests)
+- Aim for high test coverage of public APIs
+- Write unit tests that are isolated and fast
+- Use meaningful test method names that describe what is being tested
+
+## Dependencies and Packages
+
+- Target framework: **.NET 9.0**
+- Keep dependencies up to date
+- Add new dependencies thoughtfully, considering maintenance and security
+
+## Best Practices
+
+1. **Code Quality**
+   - Run `dotnet format` before committing
+   - Ensure all tests pass before submitting PRs
+   - Follow existing patterns in the codebase
+
+2. **Security**
+   - Never commit secrets, API keys, or credentials
+   - Use secure coding practices for authentication and data handling
+   - Validate and sanitize all external inputs
+
+3. **Performance**
+   - Consider async/await patterns for I/O operations
+   - Avoid blocking calls in async methods
+   - Use appropriate data structures for the task
+
+4. **Maintainability**
+   - Keep methods focused and single-purpose
+   - Extract complex logic into well-named helper methods
+   - Add comments for non-obvious business logic
+
+## Common Tasks
+
+### Adding a New Library
+1. Create project in `Libraries/` directory
+2. Follow existing project structure and naming
+3. Add corresponding test project in `Tests/`
+4. Update solution file if needed
+5. Add README.md with package documentation
+
+### Adding a New Sample
+1. Create project in `Samples/` directory
+2. Include clear README with setup instructions
+3. Demonstrate best practices for SDK usage
+4. Keep samples simple and focused
+
+### Making API Changes
+1. Maintain backward compatibility when possible
+2. Mark obsolete APIs with `[Obsolete]` attribute before removal
+3. Update XML documentation
+4. Add/update tests for new functionality
+5. Update relevant README files
+
+## CI/CD
+
+The repository uses GitHub Actions for continuous integration:
+- **build-test-lint.yml** - Runs on PRs and pushes to main
+- **codeql.yml** - Security scanning
+- All changes must pass CI checks before merging
+
+## Contact
+
+For questions or feedback: TeamsAISDKFeedback@microsoft.com