Feature Request: Add subagent information to statusline JSON input

Feature Request

Description:
Add subagent/Task tool context to the JSON data passed to custom statusline scripts.

Use Case:
I'd like to display the number of currently active subagents in my statusline (e.g., between the model name and user name). This would give visibility into background work happening during a session.

Suggested Data to Include:

• Count of active subagents
• List of running subagent IDs and their types
• Subagent status (running, completed, etc.)

Current Limitation:
The statusline JSON input includes model info, token usage, cost metrics, and session data, but has no visibility into subagent operations. The only workaround is parsing the transcript file, which only shows historical usage rather than currently active agents.

Example Use:

Opus 4.5 | 2 subagents | jeremy

This would be valuable for users who frequently use the Task tool and want to monitor parallel agent activity.