chore: scaffold Astro Starlight project

- Astro 6.3 + Starlight 0.39
- TypeScript strict
- Sitemap integration
- Inter + JetBrains Mono variable fonts
- Custom .gitignore (no AI artifacts; no .env)
- .editorconfig

tsconfig.json

@@ -0,0 +1,5 @@
+{
+  "extends": "astro/tsconfigs/strict",
+  "include": [".astro/types.d.ts", "**/*"],
+  "exclude": ["dist"]
+}