Add per-topic structured fact-cache pattern, validator, and docs

Introduce a machine-readable layer on top of the markdown corpus so AI/scripts
can query a topic's facts without re-reading whole sources (anti-RAG stays for
synthesis/quotes).

- md/demonology/demons.json: fact-cache, 33 entities attested in 2+ sources,
  each with rank/domain/signs/origins + provenance (sources, citations).
- md/demonology/demons.schema.json: JSON Schema for the dataset.
- md/demonology/INDEX.md: topic front-door (query JSON -> synthesis -> source).
- validate.py: generic schema + house-rule validator (source_count, cross_refs,
  unique ids); discovers <name>.schema.json/<name>.json pairs across all topics.
- docs/data-convention.md: the reusable, topic-agnostic pattern + how to add it
  to a new topic.
- CLAUDE.md: pointer so the convention is picked up every session.
- requirements.txt: add jsonschema (used by validate.py).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -49,6 +49,24 @@ When asked to synthesize a topic:
    `md/<topic>/<slug>-synthesis.md`, unless told otherwise.
 4. Syntheses are part of the record — they get committed.
 
+## Structured data (fact-caches)
+
+A topic may carry a machine-readable **fact-cache** alongside its markdown so you
+can answer attribute questions without re-reading the corpus. Convention, per
+topic folder `md/<topic>/`:
+
+- `<name>.json` — facts, one record per entity, each with `sources` +
+  `citations` back to the source files.
+- `<name>.schema.json` — JSON Schema; validates the sibling `<name>.json`.
+- `INDEX.md` — read this **first**: query the `.json` for facts → the
+  `*-synthesis.md` for narrative → a source `.md` only for verbatim quotes.
+
+Validate after any edit: `./.venv/bin/python validate.py <topic>` (or no arg for
+the whole repo). Edit the **JSON first**, validate, then update prose.
+
+Full pattern + how to add it to a new topic: **`docs/data-convention.md`**.
+Worked example: `md/demonology/`.
+
 ## Git conventions
 
 - Treat this repo as code: **everything goes in git** except what `.gitignore`