Arthur T. Demoulas is out as Market Basket president and CEO following months of escalating corporate tensions and public mudslinging, the company's board announced Wednesday. Board members voted unanimously late on Tuesday to oust the embattled leader, who had been on paid leave since May amid an investigation into allegations that he and his allies were planning a work stoppage. Specifically, the board suggested Demoulas intended to retaliate after board members demanded both access to key employees and collaboration "regarding the most basic corporate oversight."