Returns a report mapping employees to their completed superpowers (previously called "skills"). Only rows with an actual completion are emitted; one row per user × superpower pair. Use from and to to bound results by the superpower completion date (inclusive). skill_slug optionally narrows the result to a single superpower. skill_visibility is "open" for platform-wide superpowers and "customer" for superpowers scoped to the requesting customer (cross-customer leakage is impossible by construction).