run: critic can raise a run's step ceiling mid-flight (CriticHandle.MaxSteps)

Prerequisite for a full-fidelity mort agentcritic adapter (which adjusts a
healthy-but-long run's iteration budget, not just its deadline). executus's
CriticHandle was deadline+steer only; this adds the dynamic step ceiling above
an unchanged majordomo (which already exposes WithMaxStepsFunc).

- run.RunInfo += MaxIterations (the run's base ceiling, so a critic can raise it
  relative to the baseline).
- run.CriticHandle += MaxSteps() int — polled by the executor each step via
  agent.WithMaxStepsFunc; <=0 defers to the base. The executor uses
  WithMaxStepsFunc(critic.MaxSteps) when a critic is active, else WithMaxSteps.
- critic battery: handle.maxSteps (initialised from RunInfo.MaxIterations) +
  MaxSteps(); Decision gains RaiseStepsBy so an Escalator can raise the ceiling
  alongside ExtendBy. ExtendOnce default is unchanged (time-only).

Test: a critic returning MaxSteps=5 lets a base-MaxIterations=1 run complete two
tool-dispatch steps past the base ceiling. Core stays battery-free (run doesn't
import critic).

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

run/critic.go

@@ -88,6 +88,22 @@ func (b *criticBinding) recordToolStart(name, args string) {
 	}
 }
 
+// maxStepsOption returns the agent step-ceiling Option. With no critic it's a
+// fixed WithMaxSteps(base); with a critic it's a DYNAMIC WithMaxStepsFunc that
+// polls the handle each step (so the critic can raise a long run's budget),
+// falling back to base when the handle defers (MaxSteps() <= 0).
+func (b *criticBinding) maxStepsOption(base int) agent.Option {
+	if b == nil {
+		return agent.WithMaxSteps(base)
+	}
+	return agent.WithMaxStepsFunc(func() int {
+		if n := b.h.MaxSteps(); n > 0 {
+			return n
+		}
+		return base
+	})
+}
+
 // steerOptions returns the agent RunOptions that drain the critic's steer
 // messages into the loop. Empty when there is no critic.
 func (b *criticBinding) steerOptions() []agent.RunOption {