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.

lua/oil/adapters/trash.lua

@@ -0,0 +1,9 @@
+local fs = require('oil.fs')
+
+if fs.is_mac then
+  return require('oil.adapters.trash.mac')
+elseif fs.is_windows then
+  return require('oil.adapters.trash.windows')
+else
+  return require('oil.adapters.trash.freedesktop')
+end