refactor: remove vim.g.oil declarative config

Problem: the `vim.g.oil` configuration path was added prematurely.
It adds a second config entrypoint before the plugin has stabilized
enough to justify it.

Solution: remove `vim.g.oil` support from `plugin/oil.lua`,
`config.setup()`, docs, and tests. Users configure via
`require("oil").setup({})`.

lua/oil/config.lua

@@ -422,7 +422,7 @@ local M = {}
 ---@field border? string|string[] Window border
 
 M.setup = function(opts)
-  opts = opts or vim.g.oil or {}
+  opts = opts or {}
 
   local new_conf = vim.tbl_deep_extend('keep', opts, default_config)
   if not new_conf.use_default_keymaps then