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/registry.py

@@ -41,6 +41,7 @@ class ModelProviderRegistry:
         ProviderType.AZURE,  # Azure-hosted OpenAI deployments
         ProviderType.XAI,  # Direct X.AI GROK access
         ProviderType.ZEN,  # OpenCode Zen curated models
+        ProviderType.ANTIGRAVITY,  # Antigravity unified gateway (Claude/Gemini via Google OAuth)
         ProviderType.DIAL,  # DIAL unified API access
         ProviderType.CUSTOM,  # Local/self-hosted models
         ProviderType.OPENROUTER,  # Catch-all for cloud models
@@ -140,6 +141,19 @@ class ModelProviderRegistry:
                 azure_endpoint=azure_endpoint,
                 api_version=azure_version,
             )
+        elif provider_type == ProviderType.ANTIGRAVITY:
+            # Antigravity uses OAuth tokens from file or env var
+            # The provider handles token loading internally
+            from pathlib import Path
+
+            # Check if refresh token is available from env or accounts file
+            accounts_file = Path.home() / ".config" / "opencode" / "antigravity-accounts.json"
+            if not api_key and not accounts_file.exists():
+                logging.debug("Antigravity: No refresh token in env and no accounts file found")
+                return None
+
+            # Initialize provider (it will load tokens internally)
+            provider = provider_class(api_key=api_key or "")
         else:
             if not api_key:
                 return None
@@ -341,6 +355,7 @@ class ModelProviderRegistry:
             ProviderType.OPENROUTER: "OPENROUTER_API_KEY",
             ProviderType.CUSTOM: "CUSTOM_API_KEY",  # Can be empty for providers that don't need auth
             ProviderType.DIAL: "DIAL_API_KEY",
+            ProviderType.ANTIGRAVITY: "ANTIGRAVITY_REFRESH_TOKEN",  # Uses OAuth refresh token, not API key
         }
 
         env_var = key_mapping.get(provider_type)