ofox-video-core: Ofox video API execution layer

Wraps the Ofox video generation API (https://api.ofox.ai/v1/videos) behind one script: submit a job, poll it to a terminal state, download the result, report the exact cost. Scenario skills (seedance-short-drama, seedance-ad-creative) call into this rather than duplicating API logic.

Safety contract (non-negotiable)