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/manual_progress.lua

@@ -1,5 +1,5 @@
 -- Manual test for minimizing/restoring progress window
-local Progress = require('canola.mutator.progress')
+local Progress = require('oil.mutator.progress')
 
 local progress = Progress.new()
 
@@ -13,7 +13,7 @@ for i = 1, 10, 1 do
   vim.defer_fn(function()
     progress:set_action({
       type = 'create',
-      url = string.format('canola:///tmp/test_%d.txt', i),
+      url = string.format('oil:///tmp/test_%d.txt', i),
       entry_type = 'file',
     }, i, 10)
   end, (i - 1) * 1000)