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/trash_spec.lua

@@ -6,10 +6,10 @@ describe('freedesktop', function()
   local tmphome
   local home = vim.env.XDG_DATA_HOME
   before_each(function()
-    require('canola.config').delete_to_trash = true
+    require('oil.config').delete_to_trash = true
     tmpdir = TmpDir.new()
     tmphome = TmpDir.new()
-    package.loaded['canola.adapters.trash'] = require('canola.adapters.trash.freedesktop')
+    package.loaded['oil.adapters.trash'] = require('oil.adapters.trash.freedesktop')
     vim.env.XDG_DATA_HOME = tmphome.path
   end)
   after_each(function()
@@ -21,7 +21,7 @@ describe('freedesktop', function()
       tmphome:dispose()
     end
     test_util.reset_editor()
-    package.loaded['canola.adapters.trash'] = nil
+    package.loaded['oil.adapters.trash'] = nil
   end)
 
   it('files can be moved to the trash', function()