If you shopped at Madewell between 2013 and 2015 or at sister company J.Crew between 2015 and 2017, you’ve appreciated the work of Somsack Sikhounmuong. The designer had a much-celebrated 16-year career at the company, serving as Madewell’s head of design before replacing Jenna Lyons as chief creative officer of J.Crew. But in September 2017,…