applyFalConfig wrote the default image model to
agents.defaults.imageGenerationModel, a retired key the runtime never
reads (image generation resolves agents.defaults.mediaModels.image,
and the retired key is reported as an unrecognized dead key by
config validation). After fal onboarding, image_generate still failed
with "No image-generation model configured." until a doctor --fix
migration ran. Write mediaModels.image directly, matching the vydra
and pixverse onboarding flows.