Preheat oven to 450°.
To prepare vegetables, combine first 11 ingredients; toss well. Place vegetable mixture and garlic on a large, foil-lined baking sheet. Bake at 450° for 20 minutes or until vegetables are browned and sausages are done. Cut vegetables into 1-inch pieces. Cut sausages into 1/4-inch-thick slices.
To prepare dressing, squeeze garlic cloves into a large bowl to extract garlic pulp. Discard skins. Mash garlic pulp with a fork. Add lemon juice and next 5 ingredients (lemon juice through salt) to garlic pulp; stir well.
To prepare couscous, bring 1 1/4 cups water to a boil in a medium saucepan. Gradually stir in couscous. Remove from heat; cover and let stand for 5 minutes. Fluff couscous with a fork. Combine the couscous, chickpeas, dressing, vegetables, and sausage in a large bowl; toss gently.
Combine harissa and 2 tablespoons water in a small bowl. Spoon 2 cups couscous mixture into each of 4 serving bowls. Drizzle each serving with 1 tablespoon harissa mixture.