Brummel
5a14d9cc44
The Runway milestone fieldtest surfaced a spec_gap: `aura runs family <id>` with an unknown-but-well-formed id exits 0 with empty output, and neither the ledger nor the glossary constrained that case. The behaviour is already the documented, tested CLI contract (treat-as-empty, mirroring Registry::load's missing-store = Ok(empty)); this lifts that contract to the durable C18 note so a consumer reasoning from the spec can derive it, and records that tightening to a non-zero "no such family" exit (typo-safety) stays an available future UX choice rather than a current contract. Ratifies the spec_gap from docs/specs/0055-fieldtest-runway-real-data-seam.md.