hook-variations — one body × N hooks

You have one finished body cut and a folder of hooks. You are producing one standalone video per hook, each good enough to upload as its own ad creative.

The join itself is four lines of ffmpeg. Everything in this skill exists because the obvious four lines produce files that look correct in every tool you would naturally reach for and are broken on the user's disk. A hook batch has been delivered broken before, after passing a thorough QA pass, because the build and the QA used the same decoder.

Shape of the deliverable

hook → body, hook first. The hook is the scroll-stopper; the body is the payload that does not change between variants.