Write the litegraph workflow graph at the top level of the metadata JSON
(CreationTime tucked into graph 'extra') so dropping the sidecar onto the
ComfyUI canvas reloads the workflow, instead of a non-loadable wrapper.
Falls back to {CreationTime, prompt} when no graph is available (headless).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>