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.Worker/Prime/DailyPrepPrompt.cs

@@ -16,12 +16,18 @@ public static class DailyPrepPrompt
         "--allowedTools", CandidatesTool, SetMyDayTool,
     ];
 
-    public static string BuildPrompt(int maxTasks, DateOnly today) =>
-        ClaudeDo.Data.PromptFiles.Render(
+    public static string BuildPrompt(int maxTasks, DateOnly today, string? promptOverride = null)
+    {
+        var prompt = ClaudeDo.Data.PromptFiles.Render(
             ClaudeDo.Data.PromptKind.DailyPrep,
             new Dictionary<string, string>
             {
                 ["date"] = today.ToString("yyyy-MM-dd"),
                 ["maxTasks"] = maxTasks.ToString(),
             });
+
+        // Additive by design: a schedule's override is appended, never substituted, so a user
+        // can't accidentally break the mcp__claudedo__get_daily_prep_candidates/set_my_day flow.
+        return string.IsNullOrWhiteSpace(promptOverride) ? prompt : $"{prompt}\n\n{promptOverride}";
+    }
 }

src/ClaudeDo.Worker/Prime/PrimeRunner.cs

@@ -56,7 +56,7 @@ public sealed class PrimeRunner : IPrimeRunner
             }
 
             var today = DateOnly.FromDateTime(_clock.Now.LocalDateTime);
-            var prompt = DailyPrepPrompt.BuildPrompt(maxTasks, today);
+            var prompt = DailyPrepPrompt.BuildPrompt(maxTasks, today, schedule.PromptOverride);
             var args = DailyPrepPrompt.BuildArgs(MaxTurns);
 
             using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(ct);