The salvaged PR wired auth.py / providers.py / runtime_provider.py for
google-antigravity but never registered a ProviderProfile, so the provider
was invisible to list_providers() / the model picker / alias resolution.
Register it in the gemini model-provider plugin (alongside gemini and
google-gemini-cli) with the antigravity-pa:// scheme and aliases. Also add
@pmos69 to release.py AUTHOR_MAP (CI gate).