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.

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -50,7 +50,7 @@ body:
   - type: textarea
     attributes:
       label: 'Health check'
-      description: 'Output of `:checkhealth oil`'
+      description: 'Output of `:checkhealth canola`'
       render: text
 
   - type: textarea
@@ -71,7 +71,7 @@ body:
             {
               'barrettruth/canola.nvim',
               init = function()
-                vim.g.oil = {}
+                vim.g.canola = {}
               end,
             },
           },