Add WCF C# code style Copilot skill

[afifi-ins]

Summary

• add a repository-scoped wcf-csharp-style Copilot Agent Skill for C# implementation, reviews, tests, and .NET Framework modernization
• add focused references for code style, testing, modernization, and formatting scope
• add PowerShell helpers that discover eligible changed C# files across all three solutions
• update Copilot instructions to load the skill automatically for C# work

Formatting safety

• verification is limited to changed C# files
• automatic apply formats newly added files only by default
• existing modified files are skipped unless whole-file formatting is explicitly authorized
• generated, reference, baseline, build-output, and Arcade-managed files are excluded

Validation

• gh skill publish .github/skills --dry-run
• parsed both PowerShell helpers successfully
• exercised formatting across System.ServiceModel.sln, dotnet-svcutil.sln, and dotnet-svcutil.xmlserializer.sln
• confirmed apply mode skips modified existing files
• confirmed no existing C# source changed