fix(website): resolve accessibility violations in ported detail pages

Fixes the three axe violations surfaced by the a11y audit after the
Primer Brand redesign:

- aria-prohibited-attr: `aria-label` was set on roleless `div`s in
  InstructionDetail and PluginDetail. The "Applies to" list now uses
  list/listitem roles; plugin provenance uses a group role.
- scrollable-region-focusable: the install command `code` element
  overflows horizontally but was not keyboard reachable. Added
  tabIndex to all three render sites.
- color-contrast: the Playbook "New" label used the brand's
  success-fg on success-subtle, reaching only 4.09:1 in light mode.
  Stepped one down the same green ramp for 6.14:1. Scoped to light
  mode; dark mode already passed and its green-7 is near-black.

Also corrects the stale route list in the audit script: /hooks/,
/workflows/ and /tools/ have never existed as pages.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 80686fef-efe3-4cdd-8cd6-bfa61a5d0af6

website/scripts/a11y-audit.mjs

@@ -28,21 +28,16 @@ const routes = [
   '/agents/',
   '/instructions/',
   '/skills/',
-  '/hooks/',
-  '/workflows/',
   '/extensions/',
   '/plugins/',
-  '/tools/',
   '/contributors/',
+  '/learning-hub/',
   '/learning-hub/cookbook/',
-  '/learning-hub/github-copilot-app/',
   // Representative dedicated detail pages (one per resource type) so the audit
   // covers the shared detail layout, sidebar, install buttons, and file browser.
   '/agent/accessibility/',
   '/instruction/a11y/',
   '/skill/acquire-codebase-knowledge/',
-  '/hook/dependency-license-checker/',
-  '/workflow/daily-issues-report/',
   '/plugin/accessibility-kanban/',
   '/extension/accessibility-kanban/',
 ];