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/events/handlers/on-auth-user-signed-up.handler.test.ts

@@ -0,0 +1,15 @@
+// packages/marketing-pages/src/events/handlers/on-auth-user-signed-up.handler.test.ts
+import { describe, it, expect } from "vitest";
+import { onAuthUserSignedUpHandler } from "@/events/handlers/on-auth-user-signed-up.handler";
+
+describe("onAuthUserSignedUpHandler", () => {
+  it("returns a function (factory shape)", () => {
+    const handler = onAuthUserSignedUpHandler();
+    expect(typeof handler).toBe("function");
+  });
+
+  it("does not throw on a valid stub event", async () => {
+    const handler = onAuthUserSignedUpHandler();
+    await expect(handler({} as never)).resolves.toBeUndefined();
+  });
+});