feat(cli,tui): show time since last final agent response on the status bar (#44265)

Adds an idle clock to the context/status bar in both the prompt_toolkit CLI
and the Ink TUI: once a turn completes, a dim '✓ <elapsed>' segment shows how
long the session has been idle since the last final agent response. Hidden
while a turn is live (the per-prompt elapsed timer covers that) and before
the first turn completes.

- cli.py: track _last_turn_finished_at when the agent thread exits, surface
  it via _format_idle_since() in the snapshot, render in both the wide
  fragments path and the plain-text fallback.
- ui-tui: stamp lastTurnEndedAt when busy flips false after a live turn,
  thread it through appStatus -> StatusRule, render via a ticking IdleSince
  segment sharing the duration breakpoint/width budget.

ui-tui/src/components/appLayout.tsx

@@ -366,6 +366,7 @@ const StatusRulePane = memo(function StatusRulePane({
         cols={composer.cols}
         cwdLabel={status.cwdLabel}
         indicatorStyle={ui.indicatorStyle}
+        lastTurnEndedAt={status.lastTurnEndedAt}
         liveSessionCount={ui.liveSessionCount}
         model={ui.info?.model ?? ''}
         modelFast={ui.info?.fast || ui.info?.service_tier === 'priority'}