Adds a new extension that generates images via Google Antigravity's
image models (gemini-3-pro-image, imagen-3). Features:
- Returns images as tool result attachments for inline terminal rendering
- Configurable save modes: none, project, global, custom
- Supports env vars (PI_IMAGE_SAVE_MODE, PI_IMAGE_SAVE_DIR) and config files
- Configurable aspect ratios (1:1, 16:9, etc.)
Requires OAuth login via /login for google-antigravity provider.
