D108 — On-demand backlinks and frontmatter facets

Decision. The link graph derives inbound and outbound adjacency on demand by walking the KB with each concept's physical path. It is not persisted: vault files remain the truth, including relative links inside expanded concepts, and the modest walk avoids a second index reconciliation surface. graph_neighbors selects out, in, or both direction on that same graph.

Structured predicates belong on concept_list, which inventories existing concepts, rather than search, which ranks body text by relevance. Its small frontmatter grammar intentionally supports only ANDed exact key=value and key!=value predicates plus timestamp ranges. Regex and OR are absent: they would make bounded inventory reads harder to predict without serving the operational catalog questions this interface covers.