chore: scaffold website (Astro 6 + Starlight + Tailwind v4)

Phase A of docs/plans/20260604-adr-0042-website.md. Scaffolds the site
under website/ from the Starlight template; adds Tailwind v4 (via
@tailwindcss/vite) bridged to Starlight with @astrojs/starlight-tailwind
(src/styles/global.css + customCss). Production build is green: static
output, Pagefind search index, sharp image optimization.

Template placeholders (title, example pages, sidebar) are left for
Phase B/D. Reconciles the ADR/plan/index wording from "Astro 5" to
"Astro 6" to match the scaffolded toolchain.

website/src/content.config.ts

@@ -0,0 +1,7 @@
+import { defineCollection } from 'astro:content';
+import { docsLoader } from '@astrojs/starlight/loaders';
+import { docsSchema } from '@astrojs/starlight/schema';
+
+export const collections = {
+	docs: defineCollection({ loader: docsLoader(), schema: docsSchema() }),
+};

website/src/content/docs/guides/example.md

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+---
+title: Example Guide
+description: A guide in my new Starlight docs site.
+---
+
+Guides lead a user through a specific task they want to accomplish, often with a sequence of steps.
+Writing a good guide requires thinking about what your users are trying to do.
+
+## Further reading
+
+- Read [about how-to guides](https://diataxis.fr/how-to-guides/) in the Diátaxis framework

website/src/content/docs/index.mdx

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+---
+title: Welcome to Starlight
+description: Get started building your docs site with Starlight.
+template: splash # Remove or comment out this line to display the site sidebar on this page.
+hero:
+  tagline: Congrats on setting up a new Starlight project!
+  image:
+    file: ../../assets/houston.webp
+  actions:
+    - text: Example Guide
+      link: /guides/example/
+      icon: right-arrow
+    - text: Read the Starlight docs
+      link: https://starlight.astro.build
+      icon: external
+      variant: minimal
+---
+
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+## Next steps
+
+<CardGrid stagger>
+	<Card title="Update content" icon="pencil">
+		Edit `src/content/docs/index.mdx` to see this page change.
+	</Card>
+	<Card title="Change page layout" icon="document">
+		Delete `template: splash` in `src/content/docs/index.mdx` to display a
+		sidebar on this page.
+	</Card>
+	<Card title="Add new content" icon="add-document">
+		Add Markdown or MDX files to `src/content/docs` to create new pages.
+	</Card>
+	<Card title="Configure your site" icon="setting">
+		Edit your `sidebar` and other config in `astro.config.mjs`.
+	</Card>
+	<Card title="Read the docs" icon="open-book">
+		Learn more in [the Starlight Docs](https://starlight.astro.build/).
+	</Card>
+</CardGrid>

website/src/content/docs/reference/example.md

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+---
+title: Example Reference
+description: A reference page in my new Starlight docs site.
+---
+
+Reference pages are ideal for outlining how things work in terse and clear terms.
+Less concerned with telling a story or addressing a specific use case, they should give a comprehensive outline of what you're documenting.
+
+## Further reading
+
+- Read [about reference](https://diataxis.fr/reference/) in the Diátaxis framework

website/src/styles/global.css

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+@layer base, starlight, theme, components, utilities;
+@import '@astrojs/starlight-tailwind';
+@import 'tailwindcss/theme.css' layer(theme);
+@import 'tailwindcss/utilities.css' layer(utilities);