Add custom git commands to dotfiles repo

Include git-branch-backup, git-fix-author, git-name-stash,
git-name-stash-apply, git-rebase-on, and git-rename-branch in a new
bin/ directory. Update install.sh to symlink them into ~/bin and
document them in the README.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/git-rename-branch

@@ -0,0 +1,4 @@
+#!/bin/zsh -f
+
+git branch-backup
+git checkout $1 && git branch -m $2 && git push origin -u $2 && git push origin --delete $1