Implements the plan from a GitHub issue: loads it via /ace:plan-read, gates on the Validation audit footer's unresolved-question count, executes the work items sequentially with per-item verification, then posts the session handoff via /ace:plan-handoff. Use when a published plan exists and the user asks to implement it. Do not use when no plan has been published yet (that is /ace:plan-start), or for /ace:plan-route's implement-direct route, which deliberately has no entry-point command.
