Two documents, plus the repo-root entry point. The code is the source of truth; these explain the shape and the rationale.
/CLAUDE.md(repo root) — start here: what the product is, how to run & verify, auth, the per-kind processing flow, config params, module structure, and tests.architecture.md— the what: components, the two layers, the request→result flow, the engine pipeline, the data model, and deployment notes.design-decisions.md— the why: storage/compression (and the measured "compression ≠ token savings" finding), deterministic field confidence, the three keyframe modes, editable DB-backed prompts/schemas, and the known-issues roadmap.