feat(data): add Waiting task status and CreatedBy column

- New TaskStatus.Waiting for sequential subtask chains.
- New TaskEntity.CreatedBy column with migration AddTaskCreatedBy.
- TaskRepository.GetByCreatorAsync for filtering by creator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Models/TaskEntity.cs

@@ -10,6 +10,7 @@ public enum TaskStatus
     Planning,
     Planned,
     Draft,
+    Waiting,
 }
 
 public sealed class TaskEntity
@@ -39,6 +40,8 @@ public sealed class TaskEntity
     public string? PlanningSessionToken { get; set; }
     public DateTime? PlanningFinalizedAt { get; set; }
 
+    public string? CreatedBy { get; set; }
+
     // Navigation properties
     public ListEntity List { get; set; } = null!;
     public WorktreeEntity? Worktree { get; set; }