Gated by --cleanup flag and y/N confirmation. Walks bib.attachments
and Zotero itemAttachments + items, removes storage dirs that
contain only the .md file (non-empty dirs with foreign content are
logged and the dir is skipped, but the SQL rows still drop).
Safety pre-flight refuses to run unless src/bib/sync.py contains
_sync_notes — sanity check that re-syncing won't recreate the
attachments we're deleting.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
c9b8a51f53