Switch comment-extract output in Zotero from file attachments to notes (HTML rendered from combined.md). Spec covers: renderer, write-path change in cli/comments.py, new note-sync plumbing in bib/sync.py, and a one-shot migration to retire the existing 33,351 *.md attachments across 23,605 items. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>