Feature: Use git sparse checkout for marketplace clone to reduce disk usage

[liujacky2001]

Summary

When adding a plugin marketplace via /plugin marketplace add <repo-url>, the CLI clones the entire repository with git clone --depth 1. For repos that contain more than just plugins (e.g., source code, docs, prototypes), this wastes significant disk space.

Example:

• Full clone: 138 MB
• Actual files needed (.github/plugin/ + plugins/): ~117 KB (0.9%)

Proposal

Use git sparse checkout when cloning marketplace repos so that only the plugin-related files are fetched:

1. .github/plugin/marketplace.json — the marketplace manifest
2. Paths listed in marketplace.json plugins[].source entries — the actual plugin folders

Environment

• Copilot CLI version: 1.0.15
• OS: Windows 11