Adding a personhog RPC

This skill walks through adding a new RPC end-to-end: proto definition, code generation, Rust implementation, and client updates.

Before you start: eligibility check

Personhog serves person, distinct ID, group, group type mapping, cohort membership, and feature flag hash key override data. If the data being accessed doesn't live in one of these tables, this RPC doesn't belong in personhog: