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.

spec/tmpdir.lua

@@ -1,4 +1,4 @@
-local fs = require('oil.fs')
+local fs = require('canola.fs')
 local test_util = require('spec.test_util')
 
 ---@param path string
@@ -25,7 +25,7 @@ end
 local TmpDir = {}
 
 TmpDir.new = function()
-  local path, err = vim.loop.fs_mkdtemp('oil_test_XXXXXXXXX')
+  local path, err = vim.loop.fs_mkdtemp('canola_test_XXXXXXXXX')
   if not path then
     error(err)
   end