refactor: only create BufAdd autocmd when default_file_explorer is true

Problem: the BufAdd autocmd that hijacks directory buffers was always
created, even when default_file_explorer was false. The guard inside
maybe_hijack_directory_buffer made it a no-op, but the autocmd still
fired on every BufAdd event.

Solution: wrap the BufAdd autocmd creation and initial buffer hijack
in a config.default_file_explorer conditional. The guard inside
maybe_hijack_directory_buffer is kept as defense-in-depth.

Based on: stevearc/oil.nvim#720

lua/oil/init.lua

@@ -1397,15 +1397,6 @@ M.setup = function(opts)
       vim.w.oil_original_alternate = vim.w[parent_win].oil_original_alternate
     end,
   })
-  vim.api.nvim_create_autocmd("BufAdd", {
-    desc = "Detect directory buffer and open oil file browser",
-    group = aug,
-    pattern = "*",
-    nested = true,
-    callback = function(params)
-      maybe_hijack_directory_buffer(params.buf)
-    end,
-  })
   -- mksession doesn't save oil buffers in a useful way. We have to manually load them after a
   -- session finishes loading. See https://github.com/stevearc/oil.nvim/issues/29
   vim.api.nvim_create_autocmd("SessionLoadPost", {
@@ -1424,11 +1415,21 @@ M.setup = function(opts)
     end,
   })
 
-  local bufnr = vim.api.nvim_get_current_buf()
-  if maybe_hijack_directory_buffer(bufnr) and vim.v.vim_did_enter == 1 then
-    -- manually call load on a hijacked directory buffer if vim has already entered
-    -- (the BufReadCmd will not trigger)
-    M.load_oil_buffer(bufnr)
+  if config.default_file_explorer then
+    vim.api.nvim_create_autocmd("BufAdd", {
+      desc = "Detect directory buffer and open oil file browser",
+      group = aug,
+      pattern = "*",
+      nested = true,
+      callback = function(params)
+        maybe_hijack_directory_buffer(params.buf)
+      end,
+    })
+
+    local bufnr = vim.api.nvim_get_current_buf()
+    if maybe_hijack_directory_buffer(bufnr) and vim.v.vim_did_enter == 1 then
+      M.load_oil_buffer(bufnr)
+    end
   end
 end