clan-app: Add cancellable tasks

pkgs/clan-cli/clan_lib/api/directory.py

@@ -32,6 +32,11 @@ class FileRequest:
     initial_folder: str | None = field(default=None)
 
 
+@API.register_abstract
+def cancel_task(task_id: str) -> None:
+    """Cancel a task by its op_key."""
+
+
 @API.register_abstract
 def open_file(file_request: FileRequest) -> list[str] | None:
     """