feat(providers): add Antigravity provider for unified Claude/Gemini access

Implements a new provider that uses Google's Antigravity unified gateway API to access Claude, Gemini, and other models through a single OAuth2-authenticated endpoint.

Features:
- OAuth2 token management with automatic refresh
- Multi-account rotation for rate limit distribution
- Support for Claude Opus/Sonnet 4.5 (with/without thinking)
- Support for Gemini 2.5/3 models (Pro/Flash variants)
- Thinking mode support with configurable tokens
- Image processing support
- Dual quota pool tracking (antigravity vs gemini-cli)
- Gemini-style API request format

Authentication:
- Reads from ANTIGRAVITY_REFRESH_TOKEN env var (priority)
- Falls back to ~/.config/opencode/antigravity-accounts.json
- Automatic token refresh with retry logic
- Rate limit tracking per account and quota pool

Files added:
- providers/antigravity.py - Main provider implementation
- providers/antigravity_auth.py - OAuth token manager
- providers/registries/antigravity.py - Model registry
- conf/antigravity_models.json - Model definitions (11 models)
- docs/antigravity_provider.md - Setup and usage docs
- tests/test_antigravity_provider.py - Unit tests (14 pass)

Integration:
- Added to provider priority order after ZEN
- Registered in server.py with auto-detection
- ToS warning logged on first use

providers/__init__.py

@@ -1,5 +1,6 @@
 """Model provider abstractions for supporting multiple AI providers."""
 
+from .antigravity import AntigravityProvider
 from .azure_openai import AzureOpenAIProvider
 from .base import ModelProvider
 from .gemini import GeminiModelProvider
@@ -15,6 +16,7 @@ __all__ = [
     "ModelResponse",
     "ModelCapabilities",
     "ModelProviderRegistry",
+    "AntigravityProvider",
     "AzureOpenAIProvider",
     "GeminiModelProvider",
     "OpenAIModelProvider",