feat: Add Antigravity provider integration + fix native mode startup

- Added Antigravity AI provider with Google OAuth authentication
- New integration client (antigravity.ts) with automatic endpoint fallback
- API routes for /api/antigravity/* (models, auth-status, test, chat)
- AntigravitySettings.tsx for Advanced Settings panel
- Updated session-api.ts and session-actions.ts for provider routing
- Updated opencode.jsonc with Antigravity plugin and 11 models:
  - Gemini 3 Pro Low/High, Gemini 3 Flash
  - Claude Sonnet 4.5 (+ thinking variants)
  - Claude Opus 4.5 (+ thinking variants)
  - GPT-OSS 120B Medium

- Fixed native mode startup error (was trying to launch __nomadarch_native__ as binary)
- Native mode workspaces now skip binary launch and are immediately ready

packages/ui/src/stores/session-api.ts

@@ -249,14 +249,83 @@ async function fetchZAIProvider(): Promise<Provider | null> {
   }
 }
 
+function getStoredAntigravityToken():
+  | { access_token: string; expires_in: number; created_at: number }
+  | null {
+  if (typeof window === "undefined") return null
+  try {
+    const raw = window.localStorage.getItem(getUserScopedKey("antigravity_oauth_token"))
+    if (!raw) return null
+    return JSON.parse(raw)
+  } catch {
+    return null
+  }
+}
+
+function isAntigravityTokenValid(token: { expires_in: number; created_at: number } | null): boolean {
+  if (!token) return false
+  const createdAt = token.created_at > 1e12 ? Math.floor(token.created_at / 1000) : token.created_at
+  const expiresAt = (createdAt + token.expires_in) * 1000 - 300000
+  return Date.now() < expiresAt
+}
+
+async function fetchAntigravityProvider(): Promise<Provider | null> {
+  // Check if user is authenticated with Antigravity (Google OAuth)
+  const token = getStoredAntigravityToken()
+  if (!isAntigravityTokenValid(token)) {
+    // Not authenticated - try to fetch models anyway (they show as available but require auth)
+    try {
+      const data = await fetchJson<{ models?: Array<{ id: string; name: string; limit?: Model["limit"] }> }>(
+        "/api/antigravity/models",
+      )
+      const models = Array.isArray(data?.models) ? data?.models ?? [] : []
+      if (models.length === 0) return null
+
+      return {
+        id: "antigravity",
+        name: "Antigravity (Google OAuth)",
+        models: models.map((model) => ({
+          id: model.id,
+          name: model.name,
+          providerId: "antigravity",
+          limit: model.limit,
+        })),
+        defaultModelId: "gemini-3-pro-high",
+      }
+    } catch {
+      return null
+    }
+  }
+
+  // User is authenticated - fetch full model list
+  const data = await fetchJson<{ models?: Array<{ id: string; name: string; limit?: Model["limit"] }> }>(
+    "/api/antigravity/models",
+  )
+  const models = Array.isArray(data?.models) ? data?.models ?? [] : []
+  if (models.length === 0) return null
+
+  return {
+    id: "antigravity",
+    name: "Antigravity (Google OAuth)",
+    models: models.map((model) => ({
+      id: model.id,
+      name: model.name,
+      providerId: "antigravity",
+      limit: model.limit,
+    })),
+    defaultModelId: "gemini-3-pro-high",
+  }
+}
+
 async function fetchExtraProviders(): Promise<Provider[]> {
-  const [ollama, zen, qwen, zai] = await Promise.all([
+  const [ollama, zen, qwen, zai, antigravity] = await Promise.all([
     fetchOllamaCloudProvider(),
     fetchOpenCodeZenProvider(),
     fetchQwenOAuthProvider(),
     fetchZAIProvider(),
+    fetchAntigravityProvider(),
   ])
-  return [ollama, zen, qwen, zai].filter((provider): provider is Provider => Boolean(provider))
+  return [ollama, zen, qwen, zai, antigravity].filter((provider): provider is Provider => Boolean(provider))
 }
 
 function removeDuplicateProviders(base: Provider[], extras: Provider[]): Provider[] {