Send a worker session message by id from `vibe_spawn`/`vibe_list`. Session retains full conversation history; refer naturally ("now do the same for the other module").

Returns immediately with an ack:
- `turn`: worker idle → new turn; result self-delivers when done.
- `steered`: worker mid-turn → message injected into the running turn as live steering.
- `queued`: worker mid-turn and not currently steerable → message runs automatically as next turn.

Use for follow-ups, corrections, scope changes, review requests. NEVER re-explain prior context; session already has it.