fix(auth): declare userSignedUpEvent in signUp manifest publishes

The signUp use case calls `bus.publish(userSignedUpEvent, ...)` but its
manifest entry declared `publishes: []`. The conformance event graph was
blind to the entire auth -> marketing-pages welcome-email flow as a
result. Declaring "auth.user.signed-up" makes `pnpm conformance` surface
the event and its publisher.

packages/auth/src/feature.manifest.ts

@@ -29,7 +29,7 @@ export const authManifest = defineFeature({
     signUp: {
       mutates: true,
       audits: [],
-      publishes: [],
+      publishes: ["auth.user.signed-up"],
       consumes: [],
     },
     signOut: {