D131 — kb_status: by_status roll-up excludes unset status

Decision. kb_status gains a by_status map (status value → count), built in the same WalkConcepts pass that already produces by_type. Concepts whose frontmatter does not declare status are left out of the map entirely, not bucketed under an empty-string key — mirroring how concept_list's where predicates already treat a missing key (D108). by_status is initialized as an empty map like by_type, so a KB with no status values in use serializes it as {} rather than omitting the key.

Rationale. status is already a free-form optional OKF field that concept_list filters on exactly (D108); without an aggregate, an agent can only discover which values are in use by guessing and calling concept_list once per guess. Excluding unset status avoids a giant "" bucket dominating the map in the common case where only a minority of concepts track status — the aggregate should answer "how much of this KB is in each declared state", not restate total. No new predicate grammar, task/workflow semantics, or status-value validation is introduced: values stay owned by each KB, exactly as scoped by issue #119.