feat(pi-subagent): unified stat line with model and tps

All five TUI stat-line surfaces (inline run blocks, overlay list, detail
view, live widget, completion messages) now share one statLine() helper:

  $0.012 · ↻8 · kimi-k3 · 42 tps · 34k tok · 12s

- Cost immediately left of the turn count; live runs show cost once > $0
  (terminal rows keep the display threshold).
- Model as provider-stripped short id, elided per-task when a parallel
  run shares one model (rendered once on the header).
- tps: 30s rolling output-token rate for live runs via usage samples
  (ring buffer, cap 64) captured on existing message-usage updates;
  lifetime average for finished runs; ~0 tps renders as a stall tell.
- Width-aware truncation drops duration/tokens/tps first; cost, turns,
  and model survive.