Add `fetch --prune` to the sync?

I feel like thematically, I'd really like it if `legit sync` also included `fetch --prune` since post-sync seeing branches that have actually been removed doesn't feel very 'synced' to me. (Otherwise I'm stuck always calling `git sync; git fetch --prune` and the main appeal of legit to me is to avoid having long git commands with a ton of options.)

(Yeah, I know I can alias this if I really want to.)

Start by locating the `legit sync` command and how it invokes Git operations. Check how `git fetch --prune` should fit into that flow, then verify that syncing removes references to branches deleted from the remote.

Written by the indexing model from the issue text.