Builds the built-in (fallback) narration instructions — same semantics as the DB template:
first person, varied phrasing that continues from what was already said, no repeats.
Parameters
digest: string
The aggregated progress digest (one or more updates, oldest first).
Builds the built-in (fallback) narration instructions — same semantics as the DB template: first person, varied phrasing that continues from what was already said, no repeats.