Add --release mode for CLI-only authentication

[atani]

Summary

• Add --release flag to upload images via GitHub Releases API
• No browser or playwright-cli needed in release mode
• Useful for GHE with SAML/SSO where browser login is required each time

New Options

• --release: Use GitHub Releases for upload (no browser needed)
• --release-tag <tag>: Customize release tag name (default: gh-attach-assets)

Usage

# Release mode (CLI auth only, no browser)
gh-attach --issue 123 --image ./screenshot.png --release

# With custom release tag
gh-attach --issue 123 --image ./screenshot.png --release --release-tag my-assets

How it works

1. Creates a release tag (gh-attach-assets by default) if it doesn't exist
2. Uploads images to the release as assets
3. Uses the release download URL in comments

Test plan

• Test --release mode on github.com
• Test --release mode on GHE with SAML/SSO
• Verify backward compatibility (browser mode still works)