[Feature Request] Dismiss finished subagents and auto-prune completed entries from agent list

Bug Description
Feature request: let the orchestrator (or the agent itself) dismiss a finished subagent, and make finished subagents leave the agent list on their own. Today a completed subagent lingers as a "completed" entry in the agent list (shown by ListAgents and the peers view), which is a different list from /tasks. The docs describe only a 30-second grace in /tasks and x to clear a failed or stopped row. They document no setting, flag, command or hook to dismiss or prune a finished subagent. SubagentStop can only allow or block the stop. SendMessage to a finished agent revives it instead of removing it, and TaskStop does not remove an entry. Agents that stop at maxTurns, or that stop with background work still running, are marked partial and resumable, and the docs do not say whether their rows ever clear. Result: a long orchestration session accumulates dozens of finished-agent entries, and the exit dialog lists leftover background shells alongside real running agents, which looks alarming. Wanted: (1) a dismiss action for finished agents, available to the orchestrator as well as the user, such as a TaskStop or SendMessage option that also removes the entry; (2) an auto-prune setting for completed entries; (3) documentation of how turn-limit stops and agents with live background children are treated.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.294
• Feedback ID: 3daefc5f-adc8-4ce6-ad33-1f087c2f2410