SEOUL — South Koreans are growing more anxious about President Donald Trump’s commitment to the North Korean nuclear deal, fearing he could simply walk away, having already declared victory back home. The president has earned the respect of many people here for his unconventional diplomacy in meeting with North Korean leader Kim Jong Un, even among those who otherwise deride Trump as a “merchant” who lacks the “class” of his predecessor. But behind the surprising goodwill and relentlessly upbeat pronouncements from Seoul officials, there are creeping doubts, according to interviews this summer with current and former government advisers, a leading...