README: correct claims the code does not bear out (closes #108)

Checked every README claim against the code on next and fixed the ones
that were wrong or missing: what /metrics serves and when, what
DNSWATCHER_MAINTENANCE_MODE does, CORS on the public routes,
notification retries and the in-memory alert history, the certificate
error field and old port entries in the state file, and the Design
tree's missing files. Also corrected: CNAMEs are not followed for
watched names, the root server list is never refreshed, the NS set is
the delegation from the domain's parent zone, notification contents,
and the system resolver being used for webhooks. Code problems found
are filed separately.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-01: README checked against the code and corrected: metrics, CORS,
+  notification retries, CNAMEs, state file fields, Design tree (closes #108).
 - 2026-10-01: a certificate within the expiry warning period is warned about on
   every TLS check, where some checks used to skip it at random (closes #204).
 - 2026-10-01: a domain's NS set is its delegation from the parent zone's
@@ -127,6 +129,5 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 - 1.0 readiness: run it with a real config and read the logs:
   https://git.eeqj.de/sneak/dnswatcher/issues/66
-- README accuracy sweep: https://git.eeqj.de/sneak/dnswatcher/issues/108
 - fixed root server order: https://git.eeqj.de/sneak/dnswatcher/issues/138
 - review toward 1.0: https://git.eeqj.de/sneak/dnswatcher/issues/144