feat(librarian): conditionally enable thinking based on model type

- Add isGeminiModel helper to detect Gemini models
- Disable thinking config for Gemini models (not supported)
- Enable thinking with 32000 token budget for other models
- Add tests verifying both Gemini and Claude behavior

🤖 Generated with assistance of OhMyOpenCode

src/agents/types.ts

@@ -56,6 +56,10 @@ export function isGptModel(model: string): boolean {
   return model.startsWith("openai/") || model.startsWith("github-copilot/gpt-")
 }
 
+export function isGeminiModel(model: string): boolean {
+  return model.includes("gemini")
+}
+
 export type BuiltinAgentName =
   | "sisyphus"
   | "oracle"