Startup synthetic continue recovery

Chose a fresh synthetic queued continue, canonically derived from the lost execution with an exact-target durable wake marker, over replaying provider/tool work or scanning pending inputs because uncertain side effects must never repeat. Recover only the newest unfinished assistant: completed Step.Failed must never become synthetic continue, while completed continue-phase Steps must redrive. Reconcile unresolved tools into durable model-visible failures before removing the claim; unrelated admit-only prompts remain pending. Never call legacy SessionPrompt.