Show subagents of agents in the Agents view

Feature Request

Summary

The Agents view currently shows top-level agents but does not display their spawned subagents. We'd like subagents to appear nested under their parent agent in the view.

Current Behavior

The Agents view (FleetView) shows running agents, but subagents spawned via the Agent tool (or equivalent SDK calls) are not visible — they're essentially invisible from a monitoring perspective.

Desired Behavior

When an agent spawns subagents, those subagents should appear nested/indented under the parent agent in the Agents view, showing:

• Subagent name/description
• Current status (running, completed, failed)
• Their output or last message

Why This Matters

• Observability: Users can't see what subagents are doing or whether they've completed/failed
• Debugging: Hard to diagnose issues in multi-agent workflows without seeing the full tree
• Transparency: Complex tasks often spawn many subagents; visibility into the hierarchy helps understand what's happening and why

Use Case

When running a task that spawns multiple subagents (e.g., a feature-dev workflow with code-explorer, code-architect, and code-reviewer agents), it's currently impossible to monitor all the work happening without digging into log files.

Suggested UX

• Show a collapsible tree under each agent with its spawned subagents
• Indicate parent-child relationships (spawner → spawned)
• Show subagent status inline

This would make the Agents view much more useful for complex, multi-agent workflows.