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

.editorconfig

@@ -0,0 +1,15 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 2
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false
+
+[Makefile]
+indent_style = tab