I think the better unit to commit and work with is the prompt itself, and I think that the prompt is the thing that should be PR'd at this point, because ultimately the spec is what's important.
The fundamental problem there is the code generation step is non-deterministic. You might make a two sentence change to the prompt to fix a bug and the generation introduces two more. Generate again and everything is fine. Way too much uncertainty to have confidence in that approach.
Also, people aren't actually reading through most of the code that is generated or merged, so if there's a fear of deploying buggy code generated by AI, then I assure you that's already happening. A lot.
That said it's so trivial to do, why haven't you done that already?