chore: update stow ignore and install script logic

.stow-local-ignore

@@ -5,3 +5,4 @@ setup
 .gitmodules
 .zshenv.template
 README.md
+.gitconfig.d

install

@@ -25,7 +25,9 @@ echo
 echo "[+] Installing Oh My Zsh..."
 echo
 
-sh -c "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)"
+if [ ! -d "$HOME/.oh-my-zsh" ]; then
+  sh -c "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)"
+fi
 
 echo "Include $HOME/.config/ssh/*" >> "$HOME/.ssh/config"