Upload files to a task. Files attach at the task level (task.files, i.e. firm-uploaded files, listed in attached_files with uploaded_by=firm). Provide either a publicly accessible URL for Truss to download, or a signed_id from POST /projects/{project_id}/tasks/direct_upload after uploading the file bytes directly to cloud storage. Requires the files_write scope, plus files_read because the response includes presigned download URLs.