feat: course website (Astro Starlight)

Pages (index, disclaimer, about), lockfile, root test:site script, fix for
Starlight 0.42 sidebar config and <details> rendering in generated MDX.

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

package.json

@@ -5,6 +5,7 @@
   "license": "MIT",
   "scripts": {
     "test:tools": "node --test tools/test/*.test.mjs",
-    "check": "node tools/sync-check.mjs"
+    "check": "node tools/sync-check.mjs",
+    "test:site": "node --test site/scripts/*.test.mjs"
   }
 }