Live elapsed timer next to the activity indicator (ticks once per second
during a turn, format 2m 16s), driven by the same stream_start/stream_end
hooks as the activity indicator — so it spans the whole turn (all LLM calls,
tool calls, planning, sub-agent steps). On stream_end it syncs to the
backend's authoritative elapsed_seconds before freezing; on stream_stopped/
error/disconnect it freezes at the locally-measured value.

On stream_end the model also appends a turn_meta ChatItem rendered as a dim
"⏱ 2m 16s" line below the answer — the total time the agent spent on the
user's request. The value comes from stream_end.elapsed_seconds, already
measured by the backend over the whole turn, so the TUI needs no timer for
accuracy. stream_stopped/error do not get a metadata line (only completed
requests).

Shared format_duration(): None -> "—", <60s -> "16s", <3600 -> "2m 16s",
>=3600 -> "1h 2m".

Co-Authored-By: Claude <noreply@anthropic.com>