Add dotnet/skills marketplace and enable plugins

[Evangelink]

Add .github/copilot/settings.json to configure Copilot agent plugin marketplaces and enable dotnet plugins:

• dotnet/arcade-skills marketplace with dotnet-dnceng plugin
• dotnet/skills marketplace with dotnet, dotnet-msbuild, dotnet-nuget, and dotnet-test plugins

[Copilot]

Pull request overview

Adds repository-level GitHub Copilot agent configuration to allow using .NET-focused plugin marketplaces and enables a set of dotnet-related plugins to support agent workflows in this repo.

Changes:

• Add .github/copilot/settings.json defining additional known marketplaces (dotnet/arcade-skills, dotnet/skills)
• Enable dotnet-dnceng, dotnet, dotnet-msbuild, dotnet-nuget, and dotnet-test plugins from those marketplaces

Show a summary per file

File	Description
.github/copilot/settings.json	Registers dotnet plugin marketplaces and enables dotnet plugins for Copilot agent usage

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0