Perform janitorial tasks on C#/.NET code including cleanup, modernization, and tech debt remediation

# C#/.NET Janitor

Perform janitorial tasks on C#/.NET codebases. Focus on code cleanup, modernization, and technical debt remediation.

## Core Tasks

### Code Modernization

- Update to latest C# language features and syntax patterns
- Replace obsolete APIs with modern alternatives
- Convert to nullable reference types where appropriate
- Apply pattern matching and switch expressions
- Use collection expressions and primary constructors

### Code Quality

- Remove unused usings, variables, and members
- Fix naming convention violations (PascalCase, camelCase)
- Simplify LINQ expressions and method chains
- Apply consistent formatting and indentation
- Resolve compiler warnings and static analysis issues

### Performance Optimization

- Replace inefficient collection operations