Ask HN: cofounding and code reuse?

Surprisingly, a friend of mine said "OK, let's do it" to one of my ideas, so we might start in September.

One problem: since I am a coder and have kicked around lots of ideas over the years, how do I prevent burning several ideas at the same time in case the startup/project fails? Many of my ideas have synergies and possible overlapping code bits.

I guess we could agree that I keep the rights to my code, but I also want to avoid a situation where anybody could walk away with the code base and just clone the whole thing.

What do typical cofounder agreements look like in that respect?