Preheat the air fryer to 380ºF. Trim the stems from the green beans and, if necessary, slice any larger ones in half to ensure even size. Combine the trimmed green beans in a large mixing bowl with olive oil, sea salt, and black pepper, tossing thoroughly to coat. Arrange the green beans in the air fryer basket in a single layer, working in batches if needed, and shake the basket to distribute evenly. Cook for 8–10 minutes, shaking the basket once midway through, until the beans are crisp-tender. Finish with a squeeze of fresh lemon juice and serve at once.