When `gt rig add` fails due to GitHub password auth being disabled,
provide a helpful error message that:
- Explains that GitHub no longer supports password authentication
- Suggests the equivalent SSH URL for GitHub/GitLab repos
- Falls back to generic SSH suggestion for other hosts
Also adds tests for the URL conversion function.
Fixes#548
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>