feat(tui): team view — agent roster, inbox status, activity

[EngineerProjects]

Summary

A dedicated TUI panel to visualise the multi-agent team: who the agents
are, how many unread messages they have, and what they are currently doing.
Accessible via ctrl+p → Team.

UX

┌─ Team ──────────────────────────────────────────────┐
│  Alpha Team                             3 agents     │
│  ──────────────────────────────────────────────      │
│  ▶ Nexus      manager     ● idle       0 unread     │
│    Aria       researcher  ● working    2 unread     │
│    Kai        engineer    ● idle       1 unread     │
│  ──────────────────────────────────────────────      │
│  enter: open inbox  n: new agent  del: remove       │
└─────────────────────────────────────────────────────┘

Selecting an agent opens their inbox — a list of messages they have received
and sent, with thread view.

Acceptance criteria

• List all registered agents with nickname, role, status (idle/working), unread count
• Select agent → show their inbox (Receive + History)
• Open a message → show thread view
• n opens a "New agent" form (nickname, role, system prompt template)
• Delete agent with confirmation (also deletes their mailbox messages)
• Unread counts update in real time while TeamBus is running