feat(cli): add categories config for Antigravity auth

Add category model overrides for Gemini Antigravity authentication:
- visual: gemini-3-pro-high
- artistry: gemini-3-pro-high
- writing: gemini-3-pro-high

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

src/cli/config-manager.ts

@@ -310,6 +310,15 @@ export function generateOmoConfig(installConfig: InstallConfig): Record<string,
     config.agents = agents
   }
 
+  // Categories: override model for Antigravity auth (gemini-3-pro-preview → gemini-3-pro-high)
+  if (installConfig.hasGemini) {
+    config.categories = {
+      visual: { model: "google/gemini-3-pro-high" },
+      artistry: { model: "google/gemini-3-pro-high" },
+      writing: { model: "google/gemini-3-pro-high" },
+    }
+  }
+
   return config
 }