Swarm/multi-agent: show session name instead of raw tmux command, and auto-cleanup completed agents

Problem

When Claude Code spawns subagents (swarm/multi-agent mode), the terminal displays:

View teammates: `tmux -L claude-swarm-2187 a`

Two UX issues:

1. The banner shows a raw tmux command instead of a meaningful session name

The auto-generated claude-swarm-XXXX identifier is not informative. It would be much better to show the session name or task description so users can identify what the swarm is working on at a glance.

Expected: Something like View teammates: "fix-auth-middleware" (tmux -L claude-swarm-2187 a) — showing the session/task name prominently, with the tmux command as secondary info.

2. Completed subagents persist in the agent list indefinitely

After subagents finish their work, they continue to appear in the status area (e.g., loading-files-search, layout-structure-search shown as completed with elapsed time). This clutters the UI, especially during long sessions with many agent spawns.

Expected: Completed agents should either:

• Auto-collapse or fade out after a short delay
• Be moved to a "completed" section that can be toggled
• Be removed from the display once their results have been consumed

Screenshot

!swarm-ui

(The banner with View teammates: tmux -L claude-swarm-2187 a stays visible, and completed agents remain listed.)

Environment

• Claude Code CLI (latest)
• macOS / iTerm2