`updateDocument` previously built a fresh record from frontmatter only and called `putRecord`, silently dropping any PDS-side fields such as `bskyPostRef`. This broke the `<sequoia-comments>` web component after the first re-publish. Fix: fetch the existing record with `getRecord` before writing, spread its fields into the new record, then overwrite with the fresh frontmatter-derived values. This preserves all PDS-side fields while still updating the document content correctly. Fixes stevedylandev/sequoia#5