I’m working on an app and have created some UI screen mockups. I’d like to understand: 1. At which stage of the development lifecycle should UI designs be provided? 2. What is the best practice for sharing these designs with the spec-kit agent, given that it only accepts textual input? Currently, I have visual mockups of the screens. How can I effectively communicate these to the agent in a text-only format? Any guidance or examples would be greatly appreciated!