Add robots.txt generation and sitemap chunking

- bunext sitemap now writes public/robots.txt (configurable via the
  robots config key; robots: false or --no-robots to skip). Hand-written
  robots.txt files are never overwritten without a robots config
- Large sitemaps are split into public/sitemap-chunks/sitemap-N.xml
  (sitemap.chunk_size, default 10,000, max 50,000; 50MB per file) with
  sitemap.xml as a sitemap index; stale chunks are cleaned up
- Export bunextGenerateRobotsTxt
- Run tests in publish.sh

Co-Authored-By: Claude Opus 5.5 <[email protected]>

dist/index.d.ts

@@ -1,5 +1,6 @@
 import bunextInit from "./functions/bunext-init";
 import bunextRequestHandler from "./functions/server/bunext-req-handler";
+import generateRobotsTxt from "./utils/generate-robots-txt";
 import generateSitemap from "./utils/generate-sitemap";
 declare const bunext: {
     bunextRequestHandler: typeof bunextRequestHandler;
@@ -15,5 +16,6 @@ declare const bunext: {
     };
     bunextInit: typeof bunextInit;
     bunextGenerateSitemap: typeof generateSitemap;
+    bunextGenerateRobotsTxt: typeof generateRobotsTxt;
 };
 export default bunext;