explainer: one idea from the writing, said once

Takes something written — an article, a README, a release note, a paper — and produces a short clip of it being said out loud. The user supplies the source; this skill picks the one idea that fits, writes it as spoken words, and sends it to be generated.

This skill is a thin, scenario-specific layer over ofox-video-core. It owns the idea selection, the explainer prompt craft, the brief, the defaults 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.