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.

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -50,7 +50,7 @@ body:
   - type: textarea
     attributes:
       label: 'Health check'
-      description: 'Output of `:checkhealth canola`'
+      description: 'Output of `:checkhealth oil`'
       render: text
 
   - type: textarea
@@ -71,7 +71,7 @@ body:
             {
               'barrettruth/canola.nvim',
               init = function()
-                vim.g.canola = {}
+                vim.g.oil = {}
               end,
             },
           },