Feature request: Show subagent tool call details

[cephalin]

Summary

When subagents run, the CLI shows minimal information about what they're doing. In contrast, Copilot Chat in VS Code allows you to see and drill into the tool calls made by subagents.

Current behavior

The subagent details panel shows:

• Status (e.g., "Completed (24s)")
• ID, Type, Description
• First 10 lines of Prompt
• First 10 lines of Result

However, there's no visibility into:

• Which tools the subagent called
• Tool parameters and responses
• Intermediate steps/reasoning

Expected behavior

Add the ability to view subagent tool calls, similar to how Copilot Chat displays them. This would help users:

• Understand what the subagent actually did
• Debug issues when subagents produce unexpected results
• Learn from the subagent's approach to tasks

Additional context

Screenshot showing the current limited subagent details view:

The CLI already surfaces tool calls for the main agent - extending this to subagents would provide consistency and better observability.

[cephalin]

Here's how the custom agent runs as a subagent run looks in Copilot CLI:

and here it is when the same custom agent runs as a subagent in Copilot Chat: