run: critic parity — fuller RecordStep + cause-carrying Kill (distinct status)

Completes the run-critic seam so a host adapter (mort's agentcritic) has full
fidelity, closing the two limitations gadfly surfaced on mort #1334.

- RecordStep(iter int, resp *llm.Response): the completed step's model response
  is now passed to the critic (was index-only), so a host that records a trace
  (mort's ProgressRecorder) can show what the agent actually produced, not just
  an iteration count. The executor forwards s.Response; the battery ignores it
  (its Progress is count-based).
- CriticHandle.KillCause() error + ErrCriticKill: the executor now distinguishes
  an explicit critic KILL from a natural backstop expiry. runCtx uses a
  cause-carrying cancel (WithCancelCause + a MaxRuntime timer cancelling with
  DeadlineExceeded); the deadline-watch cancels with ErrCriticKill when
  KillCause()!=nil, else DeadlineExceeded. statusFor reads context.Cause →
  killed / timeout / cancelled are now distinct (were all "cancelled"). The
  battery sets killCause from Decision.KillReason on a Kill.

Tests: statusFor "killed" case (cause=ErrCriticKill, err=Canceled); fake handle
+ battery RecordStep/KillCause signatures. Core stays battery-free.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

critic/critic_test.go

@@ -51,7 +51,7 @@ func TestMonitorEscalatesOncePerIdlePeriodAndExtends(t *testing.T) {
 		t.Error("deadline should have been extended past the original")
 	}
 	// A fresh step re-arms; another idle period escalates again.
-	h.RecordStep(1)
+	h.RecordStep(1, nil)
 	time.Sleep(60 * time.Millisecond)
 	mu.Lock()
 	c2 := calls