Position an oven rack about 4 inches away from the broiler heat source, and preheat. Bring 1 1/2 cups water, the butter and 1/2 teaspoon salt to a simmer in a covered medium saucepan over medium-high heat. Remove from the heat, quickly stir in the couscous and let stand, covered, for 5 minutes. Fluff the couscous with a fork, stir in the dill, cover and set aside.
Meanwhile, toss the chicken, tomatoes, lemon zest and juice, olive oil, 1 teaspoon salt, oregano and 1/2 teaspoon pepper together on a rimmed baking sheet. Unfold the chicken thighs so they're covered with the marinade, and lay them as flat as possible. Broil until the chicken is cooked through and browned, 10 to 12 minutes, rotating the pan and flipping the tomatoes and chicken halfway through.
Transfer two chicken thighs and a few tomatoes to each plate. Stir the pan juices into the dilled couscous. Place 1 1/4 cups of couscous on each plate, and top with a scoop of chopped cucumbers, a dollop of yogurt and a lemon wedge.
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