Diff Artifact db docs before set to skip writes

Claude artifact database (ArtifactData): a 'set' on an existing doc is refused without if_version, and every write bumps the version and is visible to all viewers. In a periodic sync job, first 'list' the collection with out_dir (writes each doc as <out_dir>/<collection>/<id>.json, keeping big payloads out of context), then compare each computed doc to the saved file in a script and write only the ones that differ, pinning if_version from the listing. This keeps reruns idempotent and the version history clean.