Expose /sitemap.xml for Bing + IndexNow

Astro's sitemap integration emits /sitemap-index.xml + /sitemap-0.xml. Bing
Webmaster Tools and IndexNow probe /sitemap.xml literally, so a request
for the canonical name was 404'ing. Two changes:

- astro.config.mjs: add a 301 redirect /sitemap.xml → /sitemap-index.xml
  (alongside the existing / → /en/ redirect)
- public/robots.txt: list both Sitemap URLs so any crawler that reads
  robots.txt finds an entry it can use directly

After production redeploys (`docker compose up -d --build`), submit
https://nibiru-framework.com/sitemap.xml in Bing Webmaster Tools — it'll
follow the 301 and ingest the index.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/public/robots.txt

@@ -115,4 +115,9 @@ Allow: /
 Disallow: /api/
 
 # ── Sitemaps ───────────────────────────────────────────────────────────────
+# /sitemap.xml is a 301 to /sitemap-index.xml (Astro emits the index
+# automatically + one child sitemap-0.xml). Both URLs are listed so any
+# crawler that probes either path lands on the same content. Bing's
+# Webmaster Tools and IndexNow tend to look for /sitemap.xml literally.
+Sitemap: https://nibiru-framework.com/sitemap.xml
 Sitemap: https://nibiru-framework.com/sitemap-index.xml