I do work for a $1B company whose strength and differentiator from competitors is the "source code". In theory, I can copy the source code onto a disk on key and walk away.
Any programmer you'll work with will have the same potential. So? what's the logical solution? be a 1-man company forever?
You may find out that living in fear and suspicion will be bad for business as you'll always be the only employee, working for yourself, won't be able to scale or grow your company.
There are agreements and lawyers in the world. This is your safety net. Use it and start trusting people. The scumbags who will try to steal from you are a small rare minority. Most people are honest.
I encountered a case in a company I worked for 12 years ago of sales people who left the company, joined up with a competitor after stealing our leads database. As soon as we got word of it we took them to court -- in a matter of weeks we had an injunction order against them and they disbanded like leaves in the wind.
I am pretty sure a lawyer can pursue anyone who steals from you and sets up a competitor based on your hard work.
Without trust you'll never grow. Why live in fear and suspicion?