Out-of-tree extensions such as pgvector are not part of the Theseus binaries.
Declare them through the environment and the cluster installs digest-verified
prebuilt archives before the server starts:
export PG_VERSION_REQ="=17.11.0"
export PG_EXTENSIONS="vector"
export PG_EXTENSIONS_MANIFEST="https://github.com/leynos/df12-pg-extensions/releases/download/v1.0.0/manifest.json"
export PG_EXTENSIONS_MANIFEST_SHA256="<digest from manifest.json.sha256>"
CREATE EXTENSION vector then works in any database of the cluster, and
ClusterHandle::installed_extensions() reports what was installed. The hook
matches the running PostgreSQL major and the compile target and fails closed
when no verified archive exists; it never builds from source. See
docs/extensions.md for the manifest schema, the archive
rules and the failure modes.