feat(attachments): inject reference files into the run + clean up files on delete

TaskRunner appends attached files (absolute paths) to the run prompt as the
read-only Reference files section. Task and list deletes now remove the
on-disk attachment dir eagerly, and a startup AttachmentOrphanRecovery sweep
drops any attachments/<taskId>/ whose task no longer exists (covers list
cascade and planning-discard paths).

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -7,8 +7,13 @@ namespace ClaudeDo.Data.Repositories;
 public sealed class TaskRepository
 {
     private readonly ClaudeDoDbContext _context;
+    private readonly AttachmentStore _attachments;
 
-    public TaskRepository(ClaudeDoDbContext context) => _context = context;
+    public TaskRepository(ClaudeDoDbContext context, AttachmentStore? attachments = null)
+    {
+        _context = context;
+        _attachments = attachments ?? new AttachmentStore();
+    }
 
     #region CRUD
 
@@ -37,6 +42,7 @@ public sealed class TaskRepository
     public async Task DeleteAsync(string taskId, CancellationToken ct = default)
     {
         await _context.Tasks.Where(t => t.Id == taskId).ExecuteDeleteAsync(ct);
+        _attachments.DeleteTaskDir(taskId);
     }
 
     public async Task<TaskEntity?> GetByIdAsync(string taskId, CancellationToken ct = default)