Create GitHub Action to sync dotnet-test plugin from dotnet/skills

[Youssef1313]

NOTE: If a skill/agent got deleted, this automated isn't going to delete it. What we will need in future is:

1. Create a txt file with all skills/agents that are mirrored.
2. At the beginning of the workflow, the txt file is read, and all agents/skills in it are deleted.
3. Then delete the txt file itself.
4. Then re-create the txt file based on what exists in dotnet/skills, and copy the stuff.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a scheduled and manually-triggered GitHub Actions workflow to periodically sync the dotnet-test plugin content from dotnet/skills into this repo under .github/.

Changes:

• New workflow to checkout dotnet/skills with sparse checkout for plugins/dotnet-test
• Copies skills and agents content into .github/skills and .github/agents
• Opens/updates a PR automatically when changes are detected