* feat: add C# TUnit testing prompt template
- Added csharp-tunit.prompt.md with structured prompt template
- Provides testing guidelines and best practices for C# development using TUnit framework
- Includes comprehensive testing patterns for .NET projects
* docs: add TUnit entry to README.md
- Added TUnit Best Practices entry after NUnit and before XUnit
- References prompts/csharp-tunit.prompt.md with proper description
- Includes VS Code and VS Code Insiders installation badges
* move: relocate TUnit prompt to prompts directory
- Move csharp-tunit.prompt.md from root to prompts/ directory
- Follow established project organization pattern
- All prompt files should be in the prompts directory
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