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/manual_progress.lua

@@ -1,5 +1,5 @@
 -- Manual test for minimizing/restoring progress window
-local Progress = require('oil.mutator.progress')
+local Progress = require('canola.mutator.progress')
 
 local progress = Progress.new()
 
@@ -13,7 +13,7 @@ for i = 1, 10, 1 do
   vim.defer_fn(function()
     progress:set_action({
       type = 'create',
-      url = string.format('oil:///tmp/test_%d.txt', i),
+      url = string.format('canola:///tmp/test_%d.txt', i),
       entry_type = 'file',
     }, i, 10)
   end, (i - 1) * 1000)