runbooks: the migration is complete — mark it as history, not pending work

The cutover finished 2026-11-02 and this repo is public and read by members, so leaving a plan that reads like outstanding work is actively misleading. A dated banner now says it is complete and retained as the record; the STATE OF PLAY section is re-labelled as pre-cutover and marked superseded, with the one part known to be wrong (Listmonk's mail path, which moved to Scaleway TEM) named rather than silently corrected; and both files point at where live knowledge now lives — the cloudron-ops skill for procedure and co-op/docs/infrastructure.md for the member-facing description.

README.md

@@ -28,3 +28,8 @@ visible in the registrar's own list.
 Audited 2026-11-01, including the full commit history: no credentials, tokens, keys, member emails
 or account identifiers. Where a runbook needs a credential to exist (a backup passphrase, an S3
 key), it records **where the credential is held**, never the credential.
+
+
+## Completed
+
+- **[cloudron-server-migration.md](cloudron-server-migration.md)** — the move to Infomaniak, **completed 2026-11-02**. Kept as the record of how it was planned and done; current operational procedure now lives in the co-op's `cloudron-ops` skill and current member-facing description in `co-op/docs/infrastructure.md`.