BELFAST—The new leader of Northern Ireland’s Democratic Unionist Party (DUP) pledged to unite the bickering strands of unionism to fight the Brexit deal and lay the foundations for keeping the province in the United Kingdom. Edwin Poots, seen by some commentators as more aggressive in his opposition to Northern Ireland’s post-Brexit trade barriers and more conservative on social issues than his opponent, Jeffrey Donaldson, was narrowly elected leader of the region’s largest party on Friday. Northern Ireland’s agriculture minister takes over at a time of instability in the British province and unionist anger over the post-Brexit Northern Ireland protocol that installed a customs border with the rest of the United Kingdom. “The Northern Ireland protocol has proven to be a massive challenge for us and if we are to fight this, then it is for us to do that together,” Poots, a young Earth creationist who rejects the theory of …