* prune merged branches
* interactively prompt for prune
* delete remote tracking ref too
* disable selecting merged branches in TUIs
* include full list (including merged PRs) in PUT request to stacks API
* add prune to docs
* addressing review comments
* increment skill file version
* backfill ontoOldBase for deleted merged branches
* ensure upstack rebase checks immediate predecessor
* fix for stale ontoOldBase causing rebase conflicts