Nikola, a start-up electric truck company that had made a big splash on Wall Street, announced a broad alliance with General Motors in September that promised to give the company critical technology, financial support and credibility with investors.But the news was quickly overtaken by claims that Nikola had exaggerated its capabilities, and the firm’s founder resigned. Investors were left to wonder whether Nikola was indeed an automotive innovator and whether G.M. had made an embarrassing mistake in associating with the start-up.After weeks of talks, the two companies announced Monday that they would work together on a …