Bump clan-core and nixos-mailserver to 26.05 (NixOS 26.05) and adapt to
the breaking changes surfaced by nix flake check:
- mesh-hosts/clan.nix: read the new shared, instance-scoped zerotier vars
(zerotier-ip-<machine>-zerotier, zerotier-network-zerotier); admit
external members via the controller's native allowedIds.
- monitoring/server.nix: Grafana lost its built-in secret_key default;
mint one via a clan generator and pass it with $__file{}.
- dns/authoritative.nix: services.resolved.extraConfig removed -> settings.
- mail.nix: SNM cert API change (x509.useACMEHost + acme extraDomainNames)
and accounts/dkim option renames.
- docs: mesh runbook updated for the new var paths and allowedIds.
Berwn
48fcc3058b