Add missing csharp unittesting info pointers

[JanKrivanek]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

The existing csharp-dotnet-development agent was pointing to only a single unit testing framework instructions, the other 3 were orphaned (while of a same relevance as already added link to xunit)

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• Update to existing instruction, prompt, agent, plugin, or skill.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

This PR adds references to three existing C# unit testing framework prompts (NUnit, MSTest, TUnit) to the csharp-dotnet-development plugin. These prompts already exist in the repository's root-level prompts/ directory but were not previously included in this plugin, making them "orphaned" as described in the PR.

Changes:

• Added three unit testing framework prompts (NUnit, MSTest, TUnit) to the csharp-dotnet-development plugin
• Updated the plugin's item count from 8 to 11 in the plugins documentation
• Added corresponding command entries in the plugin's README

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
plugins/csharp-dotnet-development/.github/plugin/plugin.json	Added three new prompt items referencing existing root-level testing framework prompts
plugins/csharp-dotnet-development/README.md	Added command documentation for the three new unit testing prompts
docs/README.plugins.md	Updated the plugin's item count from 8 to 11 items

[aaronpowell]

Fixed up the symlinks.

What OS did you do the changes on, as it seemed that they weren't created properly as symlinks