Wrap two bricks with foil, or have a cast iron pan with a few heavy cans handy. Prepare gas or charcoal grill for high, indirect heat (coals to one side of the grill, or leave half of the burners of a gas grill off).
Place chicken on work surface, skin side up. Use your palm to press firmly on the breastbone to flatten breast.
In a small bowl mix together olive oil and harissa paste. Brush mixture all over both sides of the chicken, and season well with salt. Tuck wings under breast.
Place chicken, skin side down, on the indirect side of the grill. Set bricks or cast iron pan plus cans on top, and grill, covered, until skin is golden and crisp, 25 to 30 minutes.
Using tongs, remove bricks or pan; turn chicken and continue grilling until chicken is cooked through and an instant-read thermometer inserted into the thickest part of the thigh registers 160, about 25 to 30 minutes more.
Meanwhile, in a food processor combine parsley, cilantro, scallions, olive oil, and lemon juice. Pulse until chopped. Add skyr and season to taste with salt, pulse until smooth.