Use relative skills path in plugin.json

[lbussell]

This PR should fix this error in Claude Code:

  Error: Failed to install: Plugin has an invalid manifest file at /Users/loganbu
  ssell/.claude/plugins/cache/temp_local_1773958050889_6vvv6v/plugin.json.
  Validation errors: skills: Invalid input

[Copilot]

Pull request overview

This PR updates the dotnet-dnceng plugin manifest to use an explicit relative skills directory path, addressing Claude Code’s manifest validation error for the skills field.

Changes:

• Update plugins/dotnet-dnceng/plugin.json to set "skills": "./skills" instead of "skills": "skills".

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[lbussell]

The GitHub PR comment failed, not the validation.