Include last_activity_ts in the progress token sampled from each child.
_touch_activity ticks on every streamed chunk ('receiving stream
response'), every tool transition, and API-call start/completion — so a
child mid-stream on a long response is alive even though api_call_count
only advances when the call completes. Same liveness signal as the
compaction inactivity budget (PR #71508): if tokens are flowing it never
dies; staleness is measured from the last streamed token / tool activity
/ API call.
