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 zone's own answer, 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: README has Getting Started, Rationale and TODO sections, and its
   Architecture section is now Design, in the order policy sets (closes #173).
 - 2026-10-01: a zone's server that answers SERVFAIL or a referral leading no
@@ -123,6 +125,5 @@ 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