feature request: --clean-local flag for gh stack unstack to delete local branches

[timzchang]

After all PRs in a stack are merged, I want to clean up all stale branches in my stack locally. I don't believe there's a flag for this atm.

I currently have a macro for this but would love to have it be built in

  branches=$(gh stack view --json | jq -r '.branches[].name')                                                                                                                                                               
  gh stack unstack                                                                                                                                                                                                        
  git checkout dev && echo "$branches" | xargs git branch -D