feat(github): add GitHub repository loading extension package

Add new Atypical.VirtualFileSystem.GitHub NuGet package that enables
loading GitHub repositories directly into the VFS using Octokit.NET.

Features:
- Load repositories by owner/repo or from URL
- Configurable options (branch, subpath, file filters, size limits)
- Three loading strategies: Eager, Lazy, and MetadataOnly
- Extension methods for IVirtualFileSystem
- DI registration support
- Rate limit handling with custom exceptions
- Progress reporting callback

Also cleans up unused playwright screenshot files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: phmatray