feat(ai): add Google Cloud Code Assist provider

- Add new API type 'google-cloud-code-assist' for Gemini CLI / Antigravity auth
- Extract shared Google utilities to google-shared.ts
- Implement streaming provider for Cloud Code Assist endpoint
- Add 7 models: gemini-3-pro-high/low, gemini-3-flash, claude-sonnet/opus, gpt-oss

Models use OAuth authentication and have sh cost (uses Google account quota).
OAuth flow will be implemented in coding-agent in a follow-up.

packages/ai/src/index.ts

@@ -2,6 +2,7 @@ export * from "./agent/index.js";
 export * from "./models.js";
 export * from "./providers/anthropic.js";
 export * from "./providers/google.js";
+export * from "./providers/google-cloud-code-assist.js";
 export * from "./providers/openai-completions.js";
 export * from "./providers/openai-responses.js";
 export * from "./stream.js";