agentd top: live dashboard of all running agents

Add agentd top command - a live dashboard showing all running agents.

**Display (like Unix top but for agents):**

• List of running agents with columns:
• Run ID / Name
• Status (running, waiting for tool, etc.)
• Duration
• Tokens (in/out)
• Cost
• Current activity (1-line summary of latest log)
• Auto-refreshes

Interaction:

• Arrow keys to select an agent
• Enter to drill into that agent (shows agentd watch-style log following with summaries)
• q to quit

Complements existing commands:

• agentd list-runs - static list
• agentd watch <id> - follow one agent's logs
• agentd top - live overview of all, drill into any

Implementation notes:

• Reuse agentd list-runs data source
• TUI with brick or similar (check what agentd watch uses)
• May need to poll/aggregate stats from each agent's event log