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]>

package.json

@@ -1,6 +1,6 @@
 {
     "name": "@moduletrace/bunext",
-    "version": "1.1.21",
+    "version": "1.1.22",
     "main": "dist/index.js",
     "module": "index.ts",
     "dependencies": {