Executes a written multi-step implementation plan end to end - loads the plan, reviews it critically for gaps, works through each task with its verification steps, and reports when everything is done. This is the execution stage inside the delivery-flow workflow, entered by requests like "implement this plan", "run the plan", "execute the plan", or "work through these tasks" once delivery-flow has produced a written plan. Do not activate directly for a standalone request that has not gone through delivery-flow first.