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>

install.sh

@@ -55,6 +55,15 @@ link_file "gitconfig"         ".gitconfig"
 link_file "gitignore_global"  ".gitignore_global"
 link_file "fzf.zsh"           ".fzf.zsh"
 
+# Symlink custom git commands into ~/bin
+echo ""
+echo "=== Custom git commands ==="
+mkdir -p "$HOME/bin"
+for cmd in "$DOTFILES_DIR"/bin/git-*; do
+    name="$(basename "$cmd")"
+    link_file "bin/$name" "bin/$name"
+done
+
 echo ""
 echo "=== Symlinks complete ==="
 echo ""