Add spaghetti, zucchini, sour cream, and 1 TBSP butter (2 TBSP for 4 servings) to pot with sauce; toss to coat. Stir in half the Parmesan; season with salt and pepper. (TIP: If pasta seems dry, stir in reserved cooking water, a splash at a time, until creamy.) Divide between bowls. Top with chicken. Sprinkle with remaining Parmesan, remaining lemon zest, and a pinch of chili flakes, if desired. Serve with remaining lemon wedges on the side.