[FEATURE]: Show subagent start time and execution duration

Description:

It would be very useful to display timing information for subagent calls in the UI.

Currently, when multiple subagents are invoked, it can be difficult to understand when each subagent was started and how long it took to complete, especially when several agents are running or being delegated in sequence.

Proposed enhancement

For each subagent invocation, display:

• Invocation time — when the subagent was started.
• Execution duration — how long the subagent took to complete.
• Optionally, completion time as well, if it fits naturally into the existing UI.

For example, the UI could show something similar to:

▣ Delegators/Workers/Openai-Gpt-5.6-Luna/...    15:02:31
   Completed in 42s

▣ General                                        15:03:14
   Completed in 18s

▣ Code-Reviewers/Openai-Gpt-5.6-Luna/...         15:03:35
   Completed in 1m 07s

This would make it much easier to understand the execution flow, identify slow subagents, and debug or analyze complex multi-agent workflows.

Why this would be useful

This information would help with:

• understanding the timeline of agent/subagent execution;
• identifying slow or unexpectedly long-running subagents;
• debugging delegation and parallel execution;
• comparing execution times between different agents;
• getting a clearer picture of overall workflow performance.

The timestamp could be displayed inline with the subagent call, while the duration could be added once the subagent finishes.

Example

I think even a minimal implementation showing the start time + execution duration would already provide a lot of value.