Judges whether a UI component need should be met with an existing shadcn/ui or 21st.dev component, or requires a custom build guided by a real-app reference from Mobbin. Returns a structured verdict (use_existing | custom_build), not a list of search results. Call this whenever you are about to scaffold a new, non-trivial UI component from scratch, when you're unsure your own default output will look production-quality, or when the user references a specific app's pattern to match.