refactor: rename oil to canola across entire codebase

Problem: the codebase still used the upstream \`oil\` naming everywhere —
URL schemes, the \`:Oil\` command, highlight groups, user events, module
paths, filetypes, buffer/window variables, LuaCATS type annotations,
vimdoc help tags, syntax groups, and internal identifiers.

Solution: mechanical rename of every reference. URL schemes now use
\`canola://\` (plus \`canola-ssh://\`, \`canola-s3://\`, \`canola-sss://\`,
\`canola-trash://\`, \`canola-test://\`). The \`:Canola\` command replaces
\`:Oil\`. All highlight groups, user events, augroups, namespaces,
filetypes, require paths, type annotations, help tags, and identifiers
follow suit. The \`upstream\` remote to \`stevearc/oil.nvim\` has been
removed and the \`vim.g.oil\` deprecation shim dropped.

lua/oil/shell.lua

@@ -1,48 +0,0 @@
-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