Certification coach for Flows apps. Captures the app name, value case, persona, problem, and design intent through a structured conversation and writes App-Brief.md at the repo root. This is the FIRST step of the Flows app certification flow — run it immediately after `npx @cognite/cli apps create`, before building. Use when the user asks to start an app brief, run the certification coach, fill out the app brief, or run flows-app-brief.