Who owns the code I pay you to write?

You do, fully. When you pay us to build an extension, the source code is yours under a written assignment-of-IP clause in the contract. You can put it on your own GitHub, sell it, modify it, take it to another developer, open-source it, anything. We retain the right to reuse generic patterns (e.g. our boilerplate module skeleton, our PHPUnit test scaffolding) but never anything specific to your business logic, data, or branding.