Include effortLevel in statusline stdin JSON

Feature Request

The statusline stdin JSON currently includes model, context_window, rate_limits, and cost.
It would be helpful to also include the current effortLevel setting.

Why

• effortLevel is not reliably available from ~/.claude/settings.json because changes made via /model are not always persisted immediately.
• Models like Haiku do not support effort levels, but the stale value remains in settings.json, leading to inaccurate display.
• Including it in stdin JSON would provide the accurate, per-invocation value and allow statusline scripts to display it alongside the model name (e.g., "Opus 4.6/high").

Proposed change

Add an optional effort_level field to the statusline stdin JSON:

{
  "model": {"display_name": "Opus 4.6"},
  "effort_level": "high",
  ...
}

When effort level is not applicable (e.g., Haiku), the field can be omitted or null.