cognition.ai/timestamp stays anchored to the start event (the visible
row’s time). Set on completions whose timing the transcript needs as
well — e.g. the cloud proxy’s scripted_tools update — so clients can
end a run at its completion (mirrors client_side_completed.timestamp in
the webapp). Epoch milliseconds from proxy replay, ISO 8601 when live.
Like [ShellIdMeta], this struct is designed to be merged standalone into
any tool call’s _meta via [Meta::merge_into].