wip: course website (incomplete — Task 15 in progress)

Partial: Astro Starlight config, sync-content.mjs + 6 passing tests.
Missing: pages (index, disclaimer, about), harness page, lockfile,
verified build. See SDD ledger.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

site/astro.config.mjs

@@ -0,0 +1,28 @@
+import { defineConfig } from "astro/config";
+import starlight from "@astrojs/starlight";
+
+// The site is served at the root of its own subdomain. Set COURSE_HOST at build time
+// (e.g. COURSE_HOST=course.example.com); the default is a placeholder.
+const host = process.env.COURSE_HOST ?? "course.example.com";
+
+export default defineConfig({
+  site: `https://${host}`,
+  base: "/",
+  integrations: [
+    starlight({
+      title: "Systemic Design Course",
+      description: "A free, hands-on course on extensible systemic game design, in Rust and TypeScript.",
+      social: [
+        {
+          icon: "external",
+          label: "Source repository",
+          href: "https://gitea.lucaswinther.info/lucasw89/systemic-game-design-course",
+        },
+      ],
+      sidebar: [
+        { label: "Start here", items: ["disclaimer", "about"] },
+        { label: "Modules", autogenerate: { directory: "course" } },
+      ],
+    }),
+  ],
+});