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/AttachmentStore.cs

@@ -9,6 +9,18 @@ public sealed class AttachmentStore
     public AttachmentStore(string? root = null)
         => _root = root ?? Paths.Expand("~/.todo-app/attachments");
 
+    public string Root => _root;
+
+    public IReadOnlyList<string> EnumerateTaskIds()
+    {
+        if (!Directory.Exists(_root)) return Array.Empty<string>();
+        return Directory.GetDirectories(_root)
+            .Select(Path.GetFileName)
+            .Where(n => n is not null)
+            .Select(n => n!)
+            .ToList();
+    }
+
     public string TaskDir(string taskId)
         => Path.Combine(_root, taskId);