D25 — KB layout: data/ conceptual root, services/ carve-out in ResolvePath

Decision. The conceptual root is kb.Root/data/ (DataRoot()); ResolvePath makes an explicit carve-out for services/*.md (base = kb.Root), and WalkConcepts also walks services/ so that search/graph/lint/index see the Services. Rationale. Without the carve-out, Services would be listed by WalkConcepts but unreachable for reading via ResolvePath — a silent bug; the carve-out stays minimal (one check on the first path segment). Details: docs/data-plane.md §Filesystem layout of a KB.