Roast the chicken: Preheat the oven to 450 degrees F. Rinse the chicken and pat dry; season generously inside and out with salt and black pepper. Stuff the lemon half and herb sprigs inside the cavity, then place the chicken in a baking dish. Roast until golden brown all over and a thermometer inserted into the thickest part of the thigh registers 170 degrees F, about 1 hour, 10 minutes. Transfer to a cutting board and let cool slightly. Remove and discard the skin, then pull the meat off the bones and chop or tear into bite-size pieces; refrigerate, covered, until ready to use.
Make the mayonnaise: Combine the egg yolks, lemon juice and mustard in a large heatproof bowl set over a saucepan of barely simmering water (do not let the bowl touch the water). Cook, whisking constantly, until hot, about 2 minutes. Remove the bowl from the pan and gradually drizzle in the vegetable oil, whisking until thick and creamy. Whisk in 1 tablespoon water, the cayenne and 1/2 teaspoon salt.
Make the salad: Add the celery and leaves, the shallot, parsley, chives, tarragon and chicken to the prepared mayonnaise. Add salt, pepper and lemon juice to taste. Pull out some of the bread from the inside of the baguettes, then stuff with the chicken salad. Cut into sandwiches; serve with potato chips.