Sub-agent timer is lost after switching agents via /agent

Summary

When switching active agents with /agent, the visible timer for a running sub-agent is lost.

Steps to reproduce

1. Start a task that launches a sub-agent (so its elapsed timer is visible).
2. Switch to another agent using /agent.
3. Switch back or observe the original sub-agent entry.

Actual behavior

The sub-agent timer disappears or resets after the /agent switch.

Expected behavior

The sub-agent timer should remain attached to the same running sub-agent and continue incrementing across agent switches.

Impact

Loses execution-time visibility for long-running sub-agent work and makes progress tracking unreliable.

Notes

This appears to be a state continuity issue in the UI/session view rather than the underlying sub-agent execution itself.