EASTHAM — It was gray and blustery on First Encounter Beach, but nine teams of artists refused to let the summer end last weekend. The peal of a cowbell signaled the start of Eastham’s sand art contest, part of the town’s annual Windmill Weekend celebration. This year’s theme, “Back to the Farm,” spoke to Eastham’s history of producing oysters, turnips, and other crops. During the two-hour Saturday-morning competition, more than 100 spectators came to watch a different kind of digging take place.