`gh stack sync` should tell you what branches it wants to prune on your behalf

Here's a state where I just used `gh stack sync` after manually merging PR 2 in a stack of 7 PRs.

Everything looks good, except `Prune 1 merged branch` is a bit unclear.

I've already run `gh stack sync` after merging PR 1 too, so the branch for PR 1, `feature/sources-1-oak-cache` is already gone.

So the 1 branch to prune must be referring to `feature/sources-2-oak-source`, which makes sense, but it requires me to kind of interpret between the lines here.

It would be a bit more obvious (and safer?) if `Prune 1 merged branch` just listed the names of the branches it wants to prune somewhere. You see them after you say `Y`, but that's a bit too late 😬

Start by tracing the `gh stack sync` command's prune confirmation output and the code that lists merged branches. Update the pre-confirmation message so it names each branch that will be pruned, then verify that the names shown before confirmation match the branches removed after approval.

Written by the indexing model from the issue text.