feat(website): rebuild site on Primer Brand design prototype

Replace the Astro + Starlight site with the Brand Engineering design
prototype, ported component-for-component onto plain Astro + React
islands. The prototype is treated as the authority on markup and
styling; dynamic data is injected into its components rather than the
components being reinterpreted.

Framework:
- Remove @astrojs/starlight entirely, along with its document shell,
  search, footer and language selector. BaseLayout.astro now owns the
  document head, CSP, social meta and analytics.
- Add @primer/react-brand and @astrojs/react. Alias the package to its
  ESM build in vite.resolve, since the default CJS entrypoint breaks
  named-export detection during SSR, and mark it noExternal so its
  stylesheet imports resolve.
- Promote pagefind to an explicit devDependency; it was previously
  pulled in transitively by Starlight.

Pages: home, the five catalogs, the five detail routes, contributors,
Playbook index and articles, and the cookbook are all rendered by
ported prototype components inside a shared PageShell.

Detail pages share a DetailChassis (hero, breadcrumbs, sticky TOC with
scroll-spy, prev/next) while keeping what makes each type distinct: a
file switcher over bundled skill assets, an included-items grid and
external provenance for plugins, and a hero-scale preview for
extensions.

i18n: resolve translated Playbook entries via Astro.currentLocale
inside the shared article route instead of separate [locale] routes.
The explicit routes collided with the i18n fallback routes, so
translated articles were being shadowed by their English originals and
900 nonsensical double-locale pages were emitted.

Search: TopNavSearch now queries the Pagefind index client-side in
addition to the static resource index, merging on href and degrading
to the static index in dev, where no index has been built.

Also delete the superseded vanilla-TS page renderers and Astro
partials, which the ported React components fully replace.

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

website/public/media/duck-progress.svg

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