Per review: the `tls.certDir == null && !tls.acme.enable` derivation was
duplicated in hive-gateway, hive-tls, and hive-ci. Expose it once as a
read-only internal option `services.hyperhive.gateway.useSelfSigned` (the
gateway module's single source of truth) and have hive-tls and hive-ci
consume it instead of re-deriving.
Eval-proven: gateway.useSelfSigned is true on the self-signed default /
false with tls.certDir, and the hive-tls (HIVE_TLS_CA_PATH) + hive-ci
(NODE_EXTRA_CA_CERTS) wiring derives correctly from it.
3b8e77c1c1