To what extent can I refer to closed-access reviewer comments in my public task management workflow?
I think the other answer and comments focus too much on the legal aspects involved. There is also a community moral aspect to it. There is a good discussion in the Publons article Who owns the review.
The comments that the reviewer makes are meant to be only for the author(s) and editor. For the editor to help make a decision on acceptance of the paper and for the author(s) to either explain this decision or help improve the paper. Quoting this comments elsewhere isn't really part of the traditional process.
However, the issues raised by the reviewer need addressing in the code and you are following code practice by tracking issues using an appropriate (open) tool.
I think its important that you remember that the reviewers recommendations are just recommendations that you need to interpret in any case before you make any changes. I would recommend to make issues/tasks using your interpretations of what needs to be done. You should be able to state clearly both what needs to be done and why without quoting the reviewer at all.
This way you avoid any problems relating to copyright at all - publishing interpretations is fine. The recommendations are there to be acted on so there's no need to bother a probably already over-worked editor. More importantly this approach makes you think about what the changes are and why they are needed. This will help your understanding of the issues and how to fix them better.