Why: upstream merged prompt_modules as migration v8; the version
namespace is linear so the rebased task-sidebar work moves to v9.
Evidence: go vet + full go test ./... green; migration upgrade test now
walks v1<->v9 (up and down paths exercised for real).
Impact: fresh databases apply v8 (prompts) then v9 (conversation flags);
no released v8 conversation_flags ever existed, so no deployed database
can hold the old numbering.