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

## Summary

When adding a plugin marketplace via `/plugin marketplace add `, 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