remove vim-plug, use native vim packages for copilot

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

.vimrc

@@ -33,10 +33,6 @@ inoremap <C-j> <Esc>
 nnoremap Q :q!<CR>
 vnoremap v V
 
-call plug#begin('~/.vim/plugged')
-Plug 'github/copilot.vim'
-call plug#end()
-
 syntax on
 filetype plugin indent on
 au BufReadPost * if line("'\"") > 0 && line("'\"") <= line("$") | exe "normal! g`\"" | endif