fix: set buftype before BufEnter fires on oil buffers (#10)

Problem: oil sets buftype='acwrite' inside view.initialize(), which runs
in an async finish() callback after adapter.normalize_url(). BufEnter
fires before finish() completes, so user autocmds that check buftype on
oil buffers see an empty string instead of 'acwrite'.

Solution: set buftype='acwrite' early in load_oil_buffer() alongside the
existing early filetype='oil' assignment, before the async gap. The
redundant set in view.initialize() is harmless (idempotent).

Closes: stevearc/oil.nvim#710

lua/oil/init.lua

@@ -1082,6 +1082,7 @@ M.load_oil_buffer = function(bufnr)
     -- (e.g. ssh) because it will set up the filetype keybinds at the *beginning* of the loading
     -- process.
     vim.bo[bufnr].filetype = "oil"
+    vim.bo[bufnr].buftype = "acwrite"
     keymap_util.set_keymaps(config.keymaps, bufnr)
   end
   loading.set_loading(bufnr, true)