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.

plugin/canola.lua

@@ -1,3 +0,0 @@
-if vim.g.canola ~= nil then
-  require('canola').setup()
-end

plugin/oil.lua

@@ -0,0 +1,3 @@
+if vim.g.oil ~= nil then
+  require('oil').setup()
+end