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({})`.

lua/oil/shell.lua

@@ -0,0 +1,48 @@
+local M = {}
+
+M.run = function(cmd, opts, callback)
+  if not callback then
+    callback = opts
+    opts = {}
+  end
+  local stdout
+  local stderr = {}
+  local jid = vim.fn.jobstart(
+    cmd,
+    vim.tbl_deep_extend('keep', opts, {
+      stdout_buffered = true,
+      stderr_buffered = true,
+      on_stdout = function(j, output)
+        stdout = output
+      end,
+      on_stderr = function(j, output)
+        stderr = output
+      end,
+      on_exit = vim.schedule_wrap(function(j, code)
+        if code == 0 then
+          callback(nil, stdout)
+        else
+          local err = table.concat(stderr, '\n')
+          if err == '' then
+            err = 'Unknown error'
+          end
+          local cmd_str = type(cmd) == 'string' and cmd or table.concat(cmd, ' ')
+          callback(string.format("Error running command '%s'\n%s", cmd_str, err))
+        end
+      end),
+    })
+  )
+  local exe
+  if type(cmd) == 'string' then
+    exe = vim.split(cmd, '%s+')[1]
+  else
+    exe = cmd[1]
+  end
+  if jid == 0 then
+    callback(string.format("Passed invalid arguments to '%s'", exe))
+  elseif jid == -1 then
+    callback(string.format("'%s' is not executable", exe))
+  end
+end
+
+return M