Describe your issue

It appears that the delete branch option does not identify if the branch is from a fork and will attempt to delete a branch with the same name in the parent repository.

Further context

As an example, if I fork a repository, the PR branch will be my-fork:main. If delete branch is true, then the stale action will attempt to delete branch main in the parent (primary) repo.

The stale action should check if the closed PR branch is a fork and either skip branch deletion, or attempt to delete the branch in the fork.

Looking at the code https://github.com/actions/stale/blob/main/src/classes/issues-processor.ts#L923 the action is simply taking pullRequest.head.ref which is simply the branch name and does not give an indication if this is a fork repo branch. The payload appears to have several ways to detect if this branch is from a fork, including pullRequest.head.repo.fork: true or checking pullRequest.head.repo.owner.