Expose effort/reasoning level in statusline JSON data

Feature Request

The statusline script receives session JSON with model, context, cost, and other fields — but there's no field for the current reasoning effort level (e.g., low/medium/high or the extended thinking configuration).

Use Case

I'd like to display the current effort level next to the model name in my statusline, e.g.:

[Opus 4.6 · high] | 📦 projects | 🌿 main | ████░░░░ 48% | $1.23 | 🕐 5m 12s

This would make it easy to see at a glance whether I'm in a high-effort or low-effort mode without needing to check /config.

Suggested Implementation

Add a field like model.effort or reasoning.effort to the JSON object passed to the statusline script via stdin. Something like:

{
  "model": {
    "id": "claude-opus-4-6",
    "display_name": "Claude",
    "effort": "high"
  }
}

Current Workaround

None clean — would require a hook to cache effort level to a temp file and read it from the statusline script.