Add support for Gemini 4 Argon #33

Feature description

Add support for Google's new frontier model Gemini 4 Argon (gemini-4-argon / antigravity-gemini-4-argon) across the model registry, standalone CLI, and quota routing.

Use case

Gemini 4 Argon has been announced by Google for agentic coding and deep reasoning workflows. Wiring Argon into the Antigravity OAuth catalog enables users with access/quotas to route sessions to Argon via CLI, OpenCode, and Pi.

Proposed changes

• Add model definitions and aliases (antigravity-gemini-4-argon, gemini-4-argon) in packages/core/src/model-registry.ts.
• Configure quota group routing and header styles.
• Surface in CLI catalog and host adapters.