lemom juice • olive oil plus additional for brushing • oregano, fresh, finely chopped • garlic clove, minced • salt, divided • Coarsely ground black pepper • cherry tomatoes, cut in half • English cucumber, peeled and cut into 1/2-inch pieces • red onion, thinly sliced • large shrimp, peeled and deveined • (8-inch) pita loaves • yellow bell pepper, cut into wide strips •