Happy to share some ideas here:
a) Your client has a Lindy account, they share the login credentials with you, and you build the agent directly in their account
b) You build the agent in your own Lindy account and share the agent with your client as a User / Editor (through the Share menu)
c) You build the agent in your own Lindy account and share the agent with your client as a template that they clone into their own account