.NET: Agent skills improvments

[SergeyMenshykh]

• Tighten up the file-based skills implementation to follow the skills spec more strictly: .NET: Align skill folder discovery with spec #5078
• Support custom types for both inline and class-based skills: .NET: Support custom types in skill resource and script functions #5152
• Consider using reflection to annotate methods and properties of class-based skills, indicating whether they are resources or scripts: .NET: Support reflection for discovery of resources and scripts in class-based skills #5183
• Align directory/folder terminology: .NET: Standardize file skills terminology on 'directory' #5205