Support 'git-subdir' in plugins

[gsemet]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

VS Code

Body

Hello.

It seems copilot does not support "git-subdir" for plugins, that allow to checkout a remote project to get the skills or agent for a given plugin, but git using sparse checkout of the interesting path instead of a full clone.

Documentation: https://code.claude.com/docs/en/plugin-marketplaces#git-subdirectories

Example:

{
  "name": "my-plugin",
  "source": {
    "source": "git-subdir",
    "url": "https://github.com/acme-corp/monorepo.git",
    "path": "tools/claude-plugin"
  }
}

Usage: For instance, i now provide skills directly in tools source code, and to register these as remote skills in a single marketplace, sparce checkout is mandatory to avoid fetch the complete codebase only to fetch a few bytes...

Git sparse checkout is extremely efficient, we intensively use to avoid unnecessary transfers.

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