The prime_schedules.prompt_override column existed end-to-end but was
never populated or read. DailyPrepPrompt.BuildPrompt now takes an
optional override and appends it as an extra paragraph after the fixed
prompt (additive, never a replacement, so a user can't disable the
required get_daily_prep_candidates/set_my_day flow). PrimeRunner passes
schedule.PromptOverride through. The Prime tab in Settings now has a
multiline field per schedule wired to a new PromptOverride property on
PrimeScheduleRowViewModel (blank persists as null).
