remove vim-plug, use native vim packages for copilot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

install.zsh

@@ -56,13 +56,11 @@ done
 # create vim undo dir
 mkdir -p "$HOME/.vim/undo"
 
-# install vim-plug
-if [ ! -f "$HOME/.vim/autoload/plug.vim" ]; then
-  curl -fLo "$HOME/.vim/autoload/plug.vim" --create-dirs \
-    https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
-  echo "vim-plug: installed"
-  vim +PlugInstall +qall 2>/dev/null
-  echo "vim plugins: installed"
+# install copilot.vim
+copilot_dir="$HOME/.vim/pack/github/start/copilot.vim"
+if [ ! -d "$copilot_dir" ]; then
+  git clone https://github.com/github/copilot.vim.git "$copilot_dir" 2>/dev/null
+  echo "copilot.vim: installed"
 fi
 
 # install font