test(review): pin the task contract — runtime default briefs every worker, never persisted

Coverage note after review discussion: the branch no longer changes which workers receive the canonical task (main's #1312 behavior stands: every worker gets the user task or the built-in default). What it does change is persistence — setup writes an empty task and the built-in brief is applied at runtime only. Pin both halves so neither regresses: worker briefing (skill-bearing, skill-less, user-task-verbatim) and the runtime-only nature of the default.