[FEATURE] Have subagent output shown in a separate pane

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Agent teammate's output and activity is shown in a separate pane but not subagents.

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Proposed Solution

Show subagent activities in a separate pane, just like agent teammate

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

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Additional Context

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