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({})`.

.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
@@ -70,8 +70,8 @@ body:
           spec = {
             {
               'barrettruth/canola.nvim',
-              init = function()
-                vim.g.canola = {}
+              config = function()
+                require('oil').setup({})
               end,
             },
           },