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

server.py

@@ -392,6 +392,7 @@ def configure_providers():
         value = get_env(key)
         logger.debug(f"  {key}: {'[PRESENT]' if value else '[MISSING]'}")
     from providers import ModelProviderRegistry
+    from providers.antigravity import AntigravityProvider
     from providers.azure_openai import AzureOpenAIProvider
     from providers.custom import CustomProvider
     from providers.dial import DIALModelProvider
@@ -490,6 +491,21 @@ def configure_providers():
         else:
             logger.debug("OpenCode Zen API key is placeholder value")
 
+    # Check for Antigravity OAuth accounts
+    antigravity_token = get_env("ANTIGRAVITY_REFRESH_TOKEN")
+    antigravity_accounts_file = Path.home() / ".config" / "opencode" / "antigravity-accounts.json"
+    has_antigravity = False
+    if antigravity_token:
+        valid_providers.append("Antigravity (OAuth)")
+        has_antigravity = True
+        logger.info("Antigravity refresh token found - Claude/Gemini models via unified gateway")
+    elif antigravity_accounts_file.exists():
+        valid_providers.append("Antigravity (OAuth)")
+        has_antigravity = True
+        logger.info("Antigravity accounts file found - Claude/Gemini models via unified gateway")
+    else:
+        logger.debug("Antigravity: No refresh token or accounts file found")
+
     # Check for custom API endpoint (Ollama, vLLM, etc.)
     custom_url = get_env("CUSTOM_API_URL")
     if custom_url:
@@ -551,7 +567,13 @@ def configure_providers():
         registered_providers.append(ProviderType.ZEN.value)
         logger.debug(f"Registered provider: {ProviderType.ZEN.value}")
 
-    # 4. OpenRouter last (catch-all for everything else)
+    # 4. Antigravity (Google unified gateway for Claude/Gemini)
+    if has_antigravity:
+        ModelProviderRegistry.register_provider(ProviderType.ANTIGRAVITY, AntigravityProvider)
+        registered_providers.append(ProviderType.ANTIGRAVITY.value)
+        logger.debug(f"Registered provider: {ProviderType.ANTIGRAVITY.value}")
+
+    # 5. OpenRouter last (catch-all for everything else)
     if has_openrouter:
         ModelProviderRegistry.register_provider(ProviderType.OPENROUTER, OpenRouterProvider)
         registered_providers.append(ProviderType.OPENROUTER.value)