There is no such thing as `.jjignore` files [2], [3].
`--no-snapshot` is not a flag, neither on `jj edit` nor in general. Additionally, it doesn't make sense conceptually --- `jj edit` changes the working copy, which will then propagate changes to the selected commit every time you edit a file.
If you want to check out an old commit without amending it, you `jj new` on top of it. If you end up making changes that you don't care about, you can `jj abandon` them.