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/registries/antigravity.py

@@ -0,0 +1,56 @@
+"""Antigravity model registry for managing model configurations and aliases."""
+
+from __future__ import annotations
+
+from ..shared import ModelCapabilities, ProviderType
+from .base import CAPABILITY_FIELD_NAMES, CapabilityModelRegistry
+
+
+class AntigravityModelRegistry(CapabilityModelRegistry):
+    """Capability registry backed by ``conf/antigravity_models.json``.
+
+    The Antigravity provider accesses Claude, Gemini, and other models through
+    Google's unified gateway API. Models are split across two quota pools:
+
+    * **antigravity**: Claude models and Gemini 3 variants
+    * **gemini-cli**: Gemini 2.5 and preview models using Gemini CLI quota
+    """
+
+    # Extra fields beyond standard ModelCapabilities
+    EXTRA_KEYS = {"quota_pool"}
+
+    def __init__(self, config_path: str | None = None) -> None:
+        super().__init__(
+            env_var_name="ANTIGRAVITY_MODELS_CONFIG_PATH",
+            default_filename="antigravity_models.json",
+            provider=ProviderType.ANTIGRAVITY,
+            friendly_prefix="Antigravity ({model})",
+            config_path=config_path,
+        )
+
+    def _extra_keys(self) -> set[str]:
+        """Include quota_pool as a valid entry key."""
+        return self.EXTRA_KEYS
+
+    def _finalise_entry(self, entry: dict) -> tuple[ModelCapabilities, dict]:
+        """Build ModelCapabilities and extract extras like quota_pool."""
+        # Extract extras before building capabilities
+        extras = {}
+        quota_pool = entry.pop("quota_pool", None)
+        if quota_pool:
+            extras["quota_pool"] = quota_pool
+
+        # Build capabilities from remaining fields
+        filtered = {k: v for k, v in entry.items() if k in CAPABILITY_FIELD_NAMES}
+        filtered.setdefault("provider", ProviderType.ANTIGRAVITY)
+        filtered.setdefault("friendly_name", f"Antigravity ({entry.get('model_name', 'unknown')})")
+        capability = ModelCapabilities(**filtered)
+
+        return capability, extras
+
+    def get_quota_pool(self, model_name: str) -> str:
+        """Return the quota pool for a model ('antigravity' or 'gemini-cli')."""
+        extras = self.get_entry(model_name)
+        if extras and "quota_pool" in extras:
+            return extras["quota_pool"]
+        return "antigravity"  # Default to antigravity quota pool