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-branch-backup

@@ -0,0 +1,9 @@
+#!/bin/zsh -f
+
+CURRENT_BRANCH=$(git rev-parse --abbrev-ref HEAD)
+if [[ $CURRENT_BRANCH == HEAD ]] ; then
+    CURRENT_BRANCH=$(git rev-parse --short HEAD) # in case detached HEAD
+fi
+NEW_BRANCH=${CURRENT_BRANCH}-backup-$(date +%Y%m%d-%H%M%S)
+git checkout -b $NEW_BRANCH
+git checkout -