product-demo: one UI state to another, as a video

Two screenshots of the same interface — before and after a state change — go into one Ofox video job as the first and last frame, and the model animates the change between them.

This skill is a thin, scenario-specific layer over ofox-video-core. It owns how to capture a usable pair, the prompt for a UI transition, and the pre-generation cost estimate; ofox-video-core owns talking to the Ofox API correctly and safely (the OFOX_API_KEY handling, the no-resubmit rule, error-code mapping, download/verification, and reporting the downloaded file's absolute VIDEO_PATH). Read that skill's safety contract before using this one — it is not restated here.