Preheat oven to 425°.
Combine first 4 ingredients in a medium bowl; stir with a whisk.
Place chicken breast halves between 2 sheets of heavy-duty plastic wrap; pound each piece to an even thickness using a meat mallet or small heavy skillet. Sprinkle salt and pepper evenly over both sides of chicken.
Fold 4 (16 x 12-inch) sheets of heavy-duty aluminum foil in half crosswise. Open foil; layer 1/2 cup zucchini, 1/4 cup squash, 1/4 cup bell pepper, 2 tablespoons green onions, and 1 chicken breast half on half of each foil sheet. Spoon 3 tablespoons of coconut milk mixture over each serving. Fold foil over chicken and vegetables; tightly seal edges.
Place packets on a baking sheet. Bake at 425° for 22 minutes. Remove from oven; let stand 5 minutes. Place on plates. Unfold packets carefully, and sprinkle each serving with 1 teaspoon cilantro. Garnish with lime wedges. Serve immediately.