Get parent (rolled-up business) cluster

Get a rolled-up-business cluster: all establishments (sites) that share a company_id, plus the company's confidence + cohesion + fragility, and per-establishment summary rows.

"Parent" here is the product-facing grain (per the 2026-06-15 product-grain revision in docs/ENTITY_RESOLUTION_ROADMAP.md). A "parent" is any rolled-up business -- both the curated parent list and the auto-detected company clusters surface here. Singletons return a single establishment with confidence: null.

Response shape favors breadth (one row per establishment) over depth (per-record fields). Drill to /establishments/{id} for the records under a single site, or /employers/{id} for one record.