feat(prime): carry the action kind through the hub and validate custom prompts

src/ClaudeDo.Worker/Prime/PrimeScheduleValidation.cs

@@ -0,0 +1,14 @@
+using ClaudeDo.Data.Models;
+
+namespace ClaudeDo.Worker.Prime;
+
+public static class PrimeScheduleValidation
+{
+    /// <summary>Returns an error message, or null when the schedule is safe to persist.</summary>
+    public static string? Validate(PrimeScheduleDto dto)
+    {
+        if (dto.Kind == PrimeActionKind.Custom && string.IsNullOrWhiteSpace(dto.PromptOverride))
+            return "A custom Prime schedule needs a prompt.";
+        return null;
+    }
+}