Skill fix-dotnet-nullcheck-warnings

Goal

Fixes nullable and NullReference warnings in C# projects systematically and in a technically correct way.

When to use

• When warnings like CS8600 to CS8777 occur during the build.
• When a user explicitly wants to "fix NullReference warnings".
• When nullable annotations are inconsistent and lead to potential runtime NullReferenceExceptions.

Core principle

First determine all warnings completely, then decide per warning on a technical basis:

1. Can null actually occur from a domain point of view?
2. If yes: model nullability correctly (?) and ensure null-safe usage.
3. If no: tighten the API/types so that null is no longer possible (remove nullability, add guards), without introducing errors.