When Day One Agency opened its doors in March 2014, its main mission was to “stop the world in its tracks with stories that connect,” according to agency CEO Josh Rosenberg. That’s happening more on mobile for a lot of brands, including some of Day One’s clients: American Express, Chipotle, Nike and Victoria’s Secret. The 50-person shop, with offices in New York and Los Angeles, specializes in shaping, sharing and fueling brand narratives by crafting the best stories, choosing the right social channels to share them on and finding the best publications to cover them through the agency’s public relations arm. “In today’s mobile-first world, I think how a brand can relate, and [storytelling] really needs to be super specific and compelling,” Rosenberg said. Recently, the agency helped Chipotle with…