Combine the couscous with the stock in a saucepan. Bring to a boil, cover and turn off the heat. Allow to sit for 10 minutes, covered.
Put the garlic, olive oil, lemon juice, and herbs in a food processor and process until finely chopped. Add the pistachios and pulse until roughly chopped. Transfer to a large bowl and season with salt and pepper.
When the couscous is done, add it to the bowl and toss with the herb paste. Stir the arugula into the hot couscous and stir until wilted. Serve warm or at room temperature.