Philippe Auguin, 56, will have completed eight seasons with WNO by the time he steps down from his post. Having made his company debut in 2009 as an 11th-hour replacement for his ailing predecessor, Heinz Fricke, in a concert version of Wagner’s “Twilight of the Gods,” he has particularly excelled in Wagner, leading “Tristan and Isolde” in 2013 and Francesca Zambello’s “Ring” cycle in 2016, which counts as one of the company’s great triumphs.