Group library into topic folders (pdfs/demonology, md/demonology)

Topic-per-subfolder layout so the same workflow scales to other subjects.
Converter already mirrors structure; rerun is a no-op (idempotent).

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

README.md

@@ -18,7 +18,8 @@ demonology/
 
 Group PDFs into topic subfolders under `pdfs/` (e.g. `pdfs/angelology/`). The
 converter mirrors that structure into `md/`. A flat `pdfs/` (no subfolders) works
-too — it just produces a flat `md/`. Currently all PDFs sit directly in `pdfs/`.
+too — it just produces a flat `md/`. Current topics: `demonology/`. Add more by
+creating a new `pdfs/<topic>/` and dropping PDFs in.
 
 > **PDFs are gitignored.** They are large and copyrighted, so only the generated
 > markdown is committed. Keep your PDFs backed up outside git. To version the