feat: GitHub release assets download

[Snider]

Summary

Add ability to download release assets (binaries, checksums) from GitHub releases.

Use Case

The wallet-releases skill needs to archive wallet binaries before projects delete them.

Proposed Commands

# All releases for a repo
borg collect github releases <owner/repo>

# Specific release
borg collect github release <owner/repo> v1.0.0

# Latest only
borg collect github release <owner/repo> --latest

Output

releases/
├── v1.0.0/
│   ├── RELEASE.md (release notes)
│   ├── wallet-linux-amd64
│   ├── wallet-darwin-arm64
│   └── checksums.txt
└── v0.9.0/
    └── ...

Options

• --assets-only - skip release notes
• --pattern "*.exe" - filter by filename
• --verify-checksums - validate after download

Acceptance Criteria

• List all releases for a repo
• Download all assets from a release
• Extract release notes as markdown
• Preserve filenames and structure
• Handle large binary files efficiently

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.