Allow editor instructions to not create a PR

[cdupuis]

Can we please add a way to enable, disable the creation of PRs when running editors via plans?

[kipz]

Do you mean vs commit directly to a branch?

[cdupuis]

no, the commit goes to branch always. But with PR: false we don't raise a PR.

[johnsonr]

We just skip the very last step of raising a PR.

This is extremely useful functionality, e.g. when we want to make a change that we think will work (e.g. upgrade a library), but not waste any human's time on it in the event that the build fails. So we'd raise a PR only at that point.

[kipz]

So we'll need a feature that allows us to raise a PR from an existing branch.