Prepare the couscous according to the package directions and fluff with a fork. Meanwhile, heat 2 tablespoons of the oil in a large skillet over medium heat. Add the garlic and cook until aromatic, about 1 minute. Add the arugula and cook until just wilted, about 1 more minute. Combine the arugula and couscous in a large bowl, season with half the salt and pepper, and toss gently. In the same skillet, heat the remaining oil over medium-high heat, season the shrimp with the remaining salt and pepper, then cook 2 minutes on each side. Serve over the couscous with a squeeze of lemon.
Tip: Baby spinach makes a nice stand-in for arugula. Buy either prewashed and bagged to make this quick dinner even quicker.