feat(prime): wire prompt_override into the daily prep prompt

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).

src/ClaudeDo.Ui/ViewModels/Modals/Settings/PrimeScheduleRowViewModel.cs

@@ -21,6 +21,7 @@ public sealed partial class PrimeScheduleRowViewModel : ViewModelBase
     [ObservableProperty] private bool _sunday;
     [ObservableProperty] private TimeSpan _timeOfDay;
     [ObservableProperty] private DateTimeOffset? _lastRunAt;
+    [ObservableProperty] private string? _promptOverride;
 
     public string LastRunLabel => LastRunAt is { } v ? v.LocalDateTime.ToString("g") : "—";
 
@@ -58,6 +59,7 @@ public sealed partial class PrimeScheduleRowViewModel : ViewModelBase
         Sunday = (dto.Days & Sun) != 0;
         TimeOfDay = dto.TimeOfDay;
         LastRunAt = dto.LastRunAt;
+        PromptOverride = dto.PromptOverride;
     }
 
     public int DaysMask()
@@ -74,5 +76,6 @@ public sealed partial class PrimeScheduleRowViewModel : ViewModelBase
     }
 
     public PrimeScheduleDto ToDto() =>
-        new(Id, DaysMask(), TimeOfDay, Enabled, LastRunAt, null);
+        new(Id, DaysMask(), TimeOfDay, Enabled, LastRunAt,
+            string.IsNullOrWhiteSpace(PromptOverride) ? null : PromptOverride);
 }