feat(marketing-pages): subscribe to auth.user.signed-up

Generated handler + Payload event-task via gen event consume,
threaded through symbols / both binders / cms re-export at the
configured anchors. bus.subscribe wires the in-memory delivery in
dev-seed; the __events.auth.user.signed-up.marketing-pages Payload
task closes the production-bus loop.

Also fixes two generator-level issues found during Phase 8:
- Drop publisher prompt's `when` clause so --args can supply the
  4th positional argument (Plop limitation: --args cannot bypass
  conditional prompts). Validate runs only in consume mode.
- Switch event-task.ts.hbs from TaskConfig<"slug-string"> to
  TaskConfig<{ input: ...; output: object }> since runtime-generated
  event slugs are not keys of TypedJobs['tasks'].

packages/marketing-pages/src/di/symbols.ts

@@ -9,5 +9,6 @@ export const MARKETING_PAGES_SYMBOLS = {
   IGetPageBySlugController: Symbol.for("marketing-pages:IGetPageBySlugController"),
   IGetSiteSettingsController: Symbol.for("marketing-pages:IGetSiteSettingsController"),
   // <gen:event-handler-symbols>
+  IOnAuthUserSignedUpHandler: Symbol.for("@repo/marketing-pages/onAuthUserSignedUp"),
   // <gen:job-symbols>
 } as const;