fix(review): stop persisting the built-in task into saved profiles

Both setup paths (guided picker fallback and non-interactive first-run) wrote the built-in default brief into the saved profile's task field, making it permanently indistinguishable from a task the user wrote — which defeated the skill-workers-get-no-injected-task rule for every profile created by setup. Persist empty; the built-in brief stays a runtime fallback for skill-less workers and the judge.