Returns agency names, slugs, and which CFR titles/chapters they own.
Useful for finding which chapter corresponds to an agency's FAR supplement.

summary_only (default True) strips the `children` and most of
`cfr_references` to keep the response compact (~20 KB vs ~100 KB).
References owned by child agencies are merged into the parent row, so
chapter lookups like DFARS (chapter 2, on a DoD child agency) still work
in summary mode. Set False for the full raw payload including children.