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