Background panel: finished teammates linger as idle indefinitely, no bulk dismiss (22 agents drown out the active shell)

Environment

• Claude Code v2.1.210, macOS (darwin 25.5.0), terminal UI
• Long-running session (multi-day, heavy Agent/teammate usage)

What happened

Teammate agents spawned earlier in the session (via the Agent tool) remain in the Background panel as idle forever after their tasks are long finished and accepted. Over a few days the panel accumulated 22 agents · 1 active shell — the one thing I actually wanted to monitor (the active shell) was pushed below the fold by 21 dead entries.

Cleanup is manual and flaky

• There is no bulk dismiss / "clean up idle agents" affordance, and finished agents are never reaped automatically.
• The only way we found was sending each agent a shutdown_request via SendMessage — one handshake per agent (21 round-trips).
• One agent repeatedly ignored the pending shutdown_request and only emitted idle_notification (twice). It finally shut down only after we sent it a plain-text message spelling out the request_id and the exact shutdown_response JSON it should reply with.

Expected

Any of:

1. Auto-reap (or auto-collapse) teammates that completed their task and have been idle past some threshold;
2. A bulk "dismiss idle agents" action in the Background panel;
3. At minimum, sort/group so the active shell stays on top and idle agents collapse into one line.

Impact

Long sessions that lean on multi-agent workflows become progressively harder to monitor; the Background panel's signal-to-noise degrades to unusable, and cleanup requires protocol-level knowledge (shutdown_request/shutdown_response + request_id) that end users shouldn't need.