Remove a vim.pack plugin: vim.pack.del updates lockfile headlessly

With Neovim's built-in vim.pack (0.12+), deleting a plugin from the vim.pack.add list does not uninstall it or drop it from nvim-pack-lock.json. Run nvim --headless -c "lua vim.pack.del({'name.nvim'})" -c 'qa!' after removing it from config; it prints 'vim.pack: Removed plugin' and removes the lock entry. The name is the repo basename, not the URL.