[FEATURE] Support subdirectory path for GitHub plugin source

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using the github source type in marketplace.json, there is no way to specify a subdirectory within the repository:

{
  "name": "my-plugin",
  "source": {
    "source": "github",
    "repo": "owner/repo"
  }
}

This means one GitHub repository can only contain one plugin when referenced as an external source. While relative paths ("source": "./plugins/my-plugin") work for plugins within the same marketplace repository, external GitHub/Git references cannot target specific subdirectories.

This forces organizations to either:

1. Repository sprawl: Create separate repositories for each plugin
2. Full marketplace adoption: Clone the entire monorepo as a marketplace (not always desirable when you only need one plugin from a large collection)

Proposed Solution

Add an optional path field to the github and git (URL) source types:

GitHub source:

{
  "name": "code-formatter",
  "source": {
    "source": "github",
    "repo": "company/claude-plugins",
    "path": "plugins/code-formatter"
  }
}

Git URL source:

{
  "name": "my-plugin",
  "source": {
    "source": "url",
    "url": "https://gitlab.com/company/plugins.git",
    "path": "plugins/my-plugin"
  }
}

Benefits

• Enables monorepo patterns for plugin development
• Reduces repository sprawl in organizations
• Allows third-party marketplaces to cherry-pick specific plugins from large collections
• Aligns with common patterns in npm workspaces, Go modules, and other package ecosystems

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

_No response_

Additional Context

_No response_