Colocate skills and add project file skill

[gewarren]

No description provided.

[Copilot]

Pull request overview

This PR adds two GitHub Copilot Agent skill definitions under .github/skills/ to guide consistent maintenance of .NET API reference snippets—one focused on consolidating snippet project files and one focused on modernizing C# snippets while preserving behavior and intent.

Changes:

• Adds a snippet-project-files skill with guidance for ensuring each snippet directory builds as a single project.
• Adds a C# snippet modernization skill describing preferred syntax updates, behavioral safeguards, and validation steps.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/skills/snippet-project-files/SKILL.md	Introduces guidance for consolidating/maintaining snippet project files so snippet directories build reliably.
.github/skills/csharp-snippet-modernization/SKILL.md	Introduces modernization guidance for C# snippets, emphasizing behavior preservation and validation.

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[wadepickett]

@gewarren, this file was renamed:
.github/skills/csharp-snippet-modernization/SKILL.md

The name in the file needs to change too from name: dotnet-snippet-modernization to
name: csharp-snippet-modernization?

[wadepickett]

@gewarren Looks good. Approved. See my note on the internal name frontmater matching the file name.

[wadepickett]

Whoops, sorry, I forgot wee are using auto-merge on approval here and I simultaniously made a suggestion and approved which resulted of course in automerge as it should. @gewarren: See my note on the internal name frontmater matching the file name.