+1 to @jrockowitz and @volkswagenchick's suggestion of a visually distinguishable nudge. I would go as far as to say that if the nudges can be posted only by a subset of users (such as maintainers), the nudges should appear separate in the comment flow (similar to how we have automated issue transitions right now).
For nudges to not be editable, they have to be minimal and generic and they should mostly point to resources rather than trying to say a mouthful. I am leaning towards having them uneditable; editing opens up a can of worms around how much deviation should be allowed from the original nudge text and if it starts becoming unhelpful to everyone involved, especially to whom we intend to help.
Further, I would want the ability to select multiple nudges. For example, a nudge can cater to both a cultural difference to respond to a personal attack. Someone able to create a nudge will select all that apply so that there is only one message that goes out (which would not escalate the situation as much as multiple nudges), and all the relevant links are included.
Lastly, while I prefer nudges to not be editable, I would like a field to describe the nudge with an example. For example, if a user selects "Personal attack", they could also write quote from the offender's text to describe why it is a personal attack.