chore(scripts): retire bonsai + wtprune once skiffs `prune --apply` ships

Summary

scripts/bonsai and scripts/wtprune are the two branch/worktree pruners in this repo:

• scripts/bonsai — bash branch-pruning analyzer (local × remote × PR × worktree →
  ARESAFE/MAYBESAFE), read-only, single repo.
• scripts/wtprune — interactive pruner that removes integrated (⊂) worktrees via
  wt remove.

Both are superseded by skiffs prune --apply (indexzero/skiffs#3), which folds bonsai's
analysis and wtprune's action (including wt remove delegation, so the pre-remove /
scratch-rescue hooks still fire) into one safe, worktree-aware pass. Once that ships and is
verified, retire both scripts.

Blocked by: indexzero/skiffs#3.

Tasks

• Confirm skiffs prune --apply covers the real workflow end-to-end (a live run on a
  disposable merged branch + a merged worktree behaves correctly, and scratch content
  under gitignored paths survives via the delegated wt remove).
• git rm scripts/bonsai
• git rm scripts/wtprune
• git grep -n -iE 'bonsai|wtprune' returns nothing outside intentional history.

Notes

• ~/.scripts is a symlink to scripts/ (see dotfiles/.exports, install/setup.sh), so
  deleting the files removes them from PATH automatically — there's no installed copy or
  installer entry to clean up.
• No other tracked file references either script by name; the only occurrences are within the
  two files themselves.
• This is intentionally a separate, reversible change from the skiffs work — don't delete
  until #skiffs3 is merged and the live verification above passes.

Start by checking indexzero/skiffs#3 and verify that skiffs prune --apply covers the disposable merged-branch and merged-worktree workflow, including scratch content under gitignored paths. Then remove scripts/bonsai and scripts/wtprune, and run git grep -n -iE 'bonsai|wtprune' to confirm no non-history references remain.