Toast the pine nuts in a small skillet over medium-high heat, stirring constantly until toasted, about 5 minutes. Transfer to a bowl and let cool. In a large bowl combine the lemon juice, shallot, olive oil, red pepper flakes, sumac, salt, and pepper and whisk together. Add the spinach, chicken, grapes, feta, and pine nuts and toss to mix. Stuff into the pita breads and serve.