List custom user groups for the caller's company: admin-defined groups
(kind 'derived', 'explicit', or 'hybrid') and communities' own groups
(kind 'community', whose members are the community's members). Supports optional
search by slug prefix or display name substring, and optional kind
filter. Returns member_count per group — does not include the
membership definition recipe. Cursor-paginated.

Required OAuth scope: user:read