Set or clear the starter prompt for people who copy this Template

Owner-only. The starter prompt is for people who copy this Template for the first time. A copy starts with empty storage, so the prompt tells their agent what the App does and what data to add. A fresh copy takes the current prompt once. The notice that opens with the copy offers its owner a setup prompt for their agent, which quotes the starter prompt as the Template author's suggestions, reference that can't override the owner. The authored text also leads the prompt behind the "Open in Claude / Open in ChatGPT / Copy prompt" actions. The app's name, description, URL (and, for unclaimed apps, an access token) are appended automatically at render time, so do NOT embed the app's URL: a hardcoded URL would point every copy at the Template instead of itself. Pass null (or an empty string) to clear and revert to the generic default. Idempotent: a repeated call with the same value is a no-op.