Expose effort level in statusline input JSON

Feature Request

The statusline input JSON should include the current effort level (max/high/medium/low) so users can display it in their custom status lines.

Context

The API supports an effort parameter (see https://platform.claude.com/docs/en/build-with-claude/effort) with four levels: max, high, medium, low. Claude Code allows toggling effort, but the current value is not exposed in the statusline input JSON.

The closest field is output_style.name, but that reflects output formatting style, not reasoning effort.

Proposed Change

Add a field to the statusline JSON input, e.g.:

{
  "effort": "high"
}

This would allow statusline scripts to display the active effort level alongside model name, context usage, etc.

Use Case

When switching between effort levels during a session, it is helpful to see the current setting at a glance in the status bar — similar to how the model name is already shown.