CHANGES
- Support 'fabric-ai' alias across Zsh, Bash, and Fish
- Use invoked command for dynamic completion list queries
- Refactor Fish completions into reusable registrar for multiple commands
- Update Bash completion to reference executable via COMP_WORDS[0]
- Extend Zsh compdef to register fabric and fabric-ai
- Add cross-shell installer script with autodetection and dry-run mode
- Document installation, features, troubleshooting in new completions guide