v1.9.0 — service-conventions joins the family

A minor release. The repository list names guestgraph/service-conventions, the code-level rules of the guestgraph services: one list of what every service has, one directory per stack, vendored by every service at a pinned release. The re-sync order places it before the engine and the connector, which both vendor it.

Nothing breaks. A member takes the release by moving the tag in conventions.json and in the uses: line of .github/workflows/conventions.yml, then sh conventions/conventions-sync sync, then its own suite.