Shred the chicken breasts. In a large, deep skillet, heat the olive oil. Add the zucchini and the remaining harissa, season with salt and pepper and cook over moderately high heat, stirring frequently, until slightly softened, about 5 minutes. Stir in the chicken and chile-tomato salad. Add the stock, season with salt and pepper and simmer just until heated through, about 3 minutes. Transfer the tagine to bowls, garnish with chopped cilantro and serve with goat yogurt and pitas.