D66 — kb_overview counts concepts, not subdirectories

Context. toolKBOverview printed for each archive DossierCount, which via ListDossiers counts subdirectories. A flat-concept KB (e.g. homelab-wiki: entities/x.md, not entities/x/) has 0 subdirectories → 0 dossiers for each archive, and an agent reads it as "empty KB" despite having dozens of healthy concepts (observed with Codex on 2026-07-05: 62 concepts, search/concept_read ok, overview at 0 dossiers).

Decision. New KB.ConceptCount(archive) recursively counts non-reserved .md files (WalkDir + okf.IsReserved, excludes index.md/log.md/_archive.md/AGENTS.md). kb_overview shows (N concepts) per archive, and (N concepts, M dossiers) when subdirectories also exist — so flat KBs and dossier-based ones both stay readable. Requires server redeploy.