feat(antigravity-auth): separate google-auth module with dynamic port allocation

- Separate Google Antigravity auth to 'oh-my-opencode/google-auth' subpath
- 'oh-my-opencode/auth' now exports OpenAI Codex auth plugin
- Implement dynamic port allocation to avoid port conflicts
- Add userAgent, requestId, sessionId fields for Antigravity API compatibility
- Add debug logging for troubleshooting (ANTIGRAVITY_DEBUG=1)

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/google-auth.ts

@@ -0,0 +1,8 @@
+import type { Plugin } from "@opencode-ai/plugin"
+import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity"
+
+const GoogleAntigravityAuthPlugin: Plugin = async (ctx) => {
+  return createGoogleAntigravityAuthPlugin(ctx)
+}
+
+export default GoogleAntigravityAuthPlugin