feat: robots.txt served file, AI bots and the non-production lock

- github.com/jimsmart/grobotstxt v1.0.3 (D-99, decision note in the
  framework repo)
- golem15_seo_robots singleton: rules and AI bot switches; eleven bots
  incl. Claude-User, all allowed by default
- GET /robots.txt: rules, a Disallow group per switched-off bot and the
  golem15.seo.robots_sitemaps lines; outside production the lock with
  X-Robots-Tag; never a 5xx (last good body, else allow-all)
- TestRobots on Google's matcher with the deciding line, and the
  noindex-blocked check (D-24)

config/config.yaml

@@ -19,3 +19,7 @@ notfound_token: ""
 # notfound_max_rows caps the 404 log; new addresses beyond it are dropped,
 # with one warning an hour.
 notfound_max_rows: 10000
+#
+# robots_sitemaps lists the sitemap addresses robots.txt names, one
+# "Sitemap:" line each, made absolute on the site URL.
+robots_sitemaps: ["/sitemap.xml"]