Fill a medium (4-quart) stockpot halfway with water; add 1 lemon half, garlic, cilantro sprigs, and jalapeno. Bring to a boil. Reduce heat to medium-low, and simmer 5 minutes. Add chicken. Simmer, partially covered, until chicken is cooked through, about 16 minutes. Using a slotted spoon, transfer chicken to a plate; discard liquid. Tent chicken with foil, and let stand until cool enough to handle. Cut chicken into 1/2-inch pieces.
Stir yogurt, mayonnaise, 2 teaspoons lemon juice from remaining lemon half, the curry powder, scallions, almonds, chopped cilantro, apple, and salt; season with pepper. Stir in chicken.
Toast pitas. Top each with a lettuce leaf. Spoon 1 cup chicken salad over lettuce, and garnish with a cilantro sprig.