fix(init): skip WinNew split handler before VimEnter (#172)

Problem: running `nvim oil-ssh://host/` from the command line sometimes
emits "Oil split could not find parent window" immediately on startup.
The `WinNew` autocmd that transfers oil window vars to new splits fires
during startup before `BufEnter` has set `oil_did_enter` on any window,
so the parent search finds nothing and warns.

Solution: guard the callback with `vim.v.vim_did_enter ~= 1` so it is
silently skipped during the pre-`VimEnter` startup phase. After
`VimEnter` the split-transfer logic runs unchanged. `vim_did_enter` is
already used in `setup()` for the same startup-completion check.

Based on: stevearc/oil.nvim#325

lua/oil/init.lua

@@ -1566,6 +1566,9 @@ M.setup = function(opts)
     pattern = '*',
     nested = true,
     callback = function(params)
+      if vim.v.vim_did_enter ~= 1 then
+        return
+      end
       local util = require('oil.util')
       if not util.is_oil_bufnr(params.buf) or vim.w.oil_did_enter then
         return