Add sitemap generation and bunext sitemap command

- Add generateSitemap util (exported as bunext.bunextGenerateSitemap)
  that builds sitemap.xml from static pages, with exclude patterns,
  additional paths (array or sync/async function), lastmod from file
  mtime, trailing slash, and default changefreq/priority options
- Let page .server.ts files export a `sitemap` function to list URLs
  for dynamic routes, or override a static page's entry
- Add `bunext sitemap` command reading `sitemap` from bunext.config.ts,
  writing public/sitemap.xml by default, with --output/--origin flags
- Fix handle-routes test leaking a grab-router module mock into
  bunext-req-handler tests; set global.BUNEXT_ROUTER instead
- Ignore /.tmp
- Document sitemap usage in README and CLAUDE.md

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

.gitignore

@@ -182,4 +182,5 @@ __fixtures__
 /.dump
 /.vscode
 /source.md
-SECURITY.md
+SECURITY.md
+/.tmp