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/preview_spec.lua

@@ -1,9 +1,9 @@
 local TmpDir = require('spec.tmpdir')
-local canola = require('canola')
+local oil = require('oil')
 local test_util = require('spec.test_util')
-local util = require('canola.util')
+local util = require('oil.util')
 
-describe('canola preview', function()
+describe('oil preview', function()
   local tmpdir
   before_each(function()
     tmpdir = TmpDir.new()
@@ -17,8 +17,8 @@ describe('canola preview', function()
 
   it('opens preview window', function()
     tmpdir:create({ 'a.txt' })
-    test_util.canola_open(tmpdir.path)
-    test_util.await(canola.open_preview, 2)
+    test_util.oil_open(tmpdir.path)
+    test_util.await(oil.open_preview, 2)
     local preview_win = util.get_preview_win()
     assert.not_nil(preview_win)
     assert(preview_win)
@@ -29,7 +29,7 @@ describe('canola preview', function()
 
   it('opens preview window when open(preview={})', function()
     tmpdir:create({ 'a.txt' })
-    test_util.canola_open(tmpdir.path, { preview = {} })
+    test_util.oil_open(tmpdir.path, { preview = {} })
     local preview_win = util.get_preview_win()
     assert.not_nil(preview_win)
     assert(preview_win)