refactor: revert canola namespace to oil and remove vim.g config (#120)

* refactor: revert module namespace from canola back to oil

Problem: the canola rename creates unnecessary friction for users
migrating from stevearc/oil.nvim — every `require('oil')` call and
config reference must change.

Solution: revert all module paths, URL schemes, autocmd groups,
highlight groups, and filetype names back to `oil`. The repo stays
`canola.nvim` for identity; the code is a drop-in replacement.

* 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({})`.

spec/tmpdir.lua

@@ -1,4 +1,4 @@
-local fs = require('canola.fs')
+local fs = require('oil.fs')
 local test_util = require('spec.test_util')
 
 ---@param path string
@@ -25,7 +25,7 @@ end
 local TmpDir = {}
 
 TmpDir.new = function()
-  local path, err = vim.loop.fs_mkdtemp('canola_test_XXXXXXXXX')
+  local path, err = vim.loop.fs_mkdtemp('oil_test_XXXXXXXXX')
   if not path then
     error(err)
   end