How to respond to allegations of misconduct in authorship dispute?
You should gather all your materials related to this project into one place (emails, codes, lab notebooks, etc) and talk to an attorney immediately. Presumably you can be dismissed from your job for research misconduct if they find against you. Only an attorney can help you navigate the related employment laws of your country, state, or region, your university's regulations and rules, and whatever granting agency's rules and regulations that you are operating under.

I would not respond until you've had an attorney competent in this area at least evaluate your case and your options.