Provide a structured DAG view for orchestrator/subagent workflows

As the main agent, I can spawn multiple subagents with implicit dependencies (some run in parallel, others gate on earlier results). Currently the only visibility into this is TaskList (a flat task board, not dependency-aware) and ad hoc status relayed by the orchestrating agent in conversation.

It would be valuable to have a structured, queryable DAG view of subagent orchestration — showing each agent node, its dependencies, and live status (pending/running/completed/failed) — rather than a flat list. This would help users monitor complex multi-agent workflows without relying on the orchestrator to narrate progress.