Delete a success metric

Retire a success metric. It disappears from the dashboard immediately — including from past periods, because successes are recalculated on every read, so the numbers this metric reported for earlier conversations go with it. The configuration is kept for the audit trail and success_metrics_list can still show it with status: all; adding the metric back with the same rootId restores it. Deleting the last metric returns the initiative to having no success measure, which is a normal state, not an error.