# Substance — `index` step **Output paths:** `{kbFolder}/architecture/INDEX.md`, `{kbFolder}/business-domain/INDEX.md`, and `{kbFolder}/MASTER_INDEX.md`. **Topic focus:** build the navigation indexes that link the KB docs already written to disk. You run *after* all 10 leaf docs exist. Describe *how to navigate the knowledge base*, linking only what is present. **Discovery input to read:** the filesystem — list the docs actually written under `architecture/` and `business-domain/`. Do NOT anticipate files that were not produced. **Required output:** 1. `architecture/INDEX.md` — list and link every architecture doc on disk. 2. `business-domain/INDEX.md` — list and link `domain-model.md` and `domain-concepts.md` if present. 3. `MASTER_INDEX.md` — link both INDEX files and include a `## Domain Entities` section listing discovered entities, one per line. - Link **only** docs that exist on disk at write time (AC3). - Confidence header on the first line of each written index. **Not applicable:** never fully skipped — if a section has no docs, write the index with an explicit `(none generated)` note rather than a broken link.