.NET: Support multiple sources for agent skills

[SergeyMenshykh]

• ADR - ADR to support a multi-source architecture for agent skills #4787
• Restructure existing agent skills API to support multiple sources: .NET: [Breaking] Restructure agent skills to use multi-source architecture #4871
  • Add file-based skill script execution.
• Support code-defined skills: .NET: Add inline skills API #4951
• Support class-based skills: .NET: skill as class #5027