Exposes the three per-source graph options in the source's retrieval settings,
shown only when the retriever is graphrag: where the walk starts (entities or
relationships), whether passages join the walk, and whether vector hits are
blended in. Defaults match the backend's measured-best configuration and are
filled in for sources saved before the options existed. A note points at the
"search tool" exposure, which is what offers the graph to an agent.
Alex
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