Put a large skillet over medium-high heat and add the grapeseed oil. When it starts to get hot, add the garlic, ginger and onions, and cook until the onions are golden, about 4 minutes. Add the tomato paste, cumin, garam masala, brown sugar, chili flakes, salt and turmeric, and cook for 2 minutes. Add the chicken and stir well to coat. Stir in the yogurt and 1/2 cup water and cook until the chicken is done, 8 to 10 minutes. Let cool, and then shred the meat using a fork or your fingers.