Add modernize-dotnet plugin

[tlmii]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow 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, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Adds the modernize-dotnet plugin as an external plugin from dotnet/modernize-dotnet.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• 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

Adds the modernize-dotnet external plugin entry so it can be discovered/installed via this repo’s plugin marketplace.

Changes:

• Added modernize-dotnet metadata to the external plugin registry (plugins/external.json).
• Regenerated/updated the consolidated plugin marketplace listing (.github/plugin/marketplace.json) to include the new external plugin.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
plugins/external.json	Registers modernize-dotnet as an external plugin (repo, metadata, and source path).
.github/plugin/marketplace.json	Adds the corresponding modernize-dotnet entry to the generated marketplace index.