Heat the oil in a large pot or Dutch oven over medium-low heat. Once oil is shimmering, add the garlic, onion, celery, season with salt and pepper and saute until softened, about 5 minutes, stirring occasionally. Add the thyme, stir for 30 seconds until fragrant, and then add the chicken broth and water and bring to a boil. Once boiling, reduce heat to medium and add the carrot noodles and chicken and let cook for 3 minutes or until the noodles are al dente. Stir in the spinach and couscous, season with salt and pepper, and let simmer for 2-3 minutes more or until spinach wilts.