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>

run/critic_test.go

@@ -23,10 +23,16 @@ type fakeCriticHandle struct {
 	mu                  sync.Mutex
 	steps, tools, stops int
 	steered             int
-	maxSteps            int // 0 => defer to the run's base MaxIterations
+	maxSteps            int   // 0 => defer to the run's base MaxIterations
+	killCause           error // non-nil simulates a critic kill
 }
 
-func (h *fakeCriticHandle) RecordStep(int) { h.mu.Lock(); h.steps++; h.mu.Unlock() }
+func (h *fakeCriticHandle) RecordStep(int, *llm.Response) { h.mu.Lock(); h.steps++; h.mu.Unlock() }
+func (h *fakeCriticHandle) KillCause() error {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	return h.killCause
+}
 func (h *fakeCriticHandle) RecordToolStart(string, string) {
 	h.mu.Lock()
 	h.tools++