Expose /usage session rate-limit data in statusline JSON

Feature Request

The /usage command shows session rate-limit usage with a progress bar and reset time, e.g.:

Current session
  █████████████████▌                                 35% used
  Resets 6pm (America/New_York)

This data is not currently available in the JSON piped to statusline scripts. It would be great to expose it so users can surface session usage directly in their status bar without having to run /usage manually.

Proposed fields

Something like:

{
  "session_usage": {
    "used_percentage": 35,
    "resets_at": "2025-09-25T22:00:00Z"
  }
}

Use case

I have a custom statusline script that already shows context window percentage (context_window.used_percentage), git status, etc. Being able to add session rate-limit usage to that bar would give a complete at-a-glance picture without interrupting the workflow to check /usage.