Cover the rice sticks with plenty of boiling water in a large bowl, soak for 5 minutes, then drain and rinse quickly under cold water. The noodles should bend without snapping but still feel firm through the center.
Whisk the tamarind puree, brown sugar, fish sauce, and oyster sauce together in a small bowl. The mixture should look dark amber and smell sweet, sharp, and savory.
Heat 2 tbsp oil in a large nonstick pan or well-seasoned skillet over high heat, then add the onion and garlic for 30 seconds. Stir until the garlic becomes fragrant and the onion edges start to soften without browning deeply.
Add the thinly sliced chicken and stir-fry for 1 1/2 minutes. The pieces should pick up light golden spots and look mostly opaque while remaining juicy.
Move the chicken to one side, add the eggs to the open side, and scramble them with a wooden spoon before folding them into the chicken. Add a touch of extra oil if the pan looks dry; the egg should form soft, separate curds rather than a dry sheet.
Add the bean sprouts, tofu, drained noodles, and prepared sauce. Keep the sprouts on top at first so they do not soften too early.
Toss gently for about 1 1/2 minutes until the noodles absorb the sauce. The noodles should turn glossy and amber, with very little loose liquid left at the bottom of the pan.
Fold in the garlic chives and half the peanuts, then remove the pan from the heat. The chives should stay bright and the peanuts should smell warm and nutty.
Serve immediately with the remaining peanuts, lime wedges, ground chilli or cayenne pepper, and more beansprouts on the side if desired. Squeeze lime over each portion just before eating for a fresh, sharp finish.