Every 30 minutes, each open document with unsaved changes since its last
version gets an automatic "Autosave" version (flagged `auto` in the file).
The newest 20 autosaves are kept; older ones make room and user versions
are never dropped for them. Runs of autosaves fold into one line in the
Version history panel. New command version.autosave.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>