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/.vscode/extensions.json

@@ -0,0 +1,4 @@
+{
+  "recommendations": ["astro-build.astro-vscode"],
+  "unwantedRecommendations": []
+}

website/.vscode/launch.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "command": "./node_modules/.bin/astro dev",
+      "name": "Development server",
+      "request": "launch",
+      "type": "node-terminal"
+    }
+  ]
+}