Feature: Expose effort level in statusline data

Feature Request

Currently, the statusline command receives a JSON object via stdin, but it does not include the current effort level (set via /effort).

Requested change:
Add an effort field to the statusline JSON data, for example:

{
  "effort": "high",   // "low" | "medium" | "high" | "auto"
  ...
}

Use case:
Users who configure a custom statusline shell command want to display the current thinking effort level alongside model name and context usage. Without this field, there is no way to reflect effort level changes in the statusline.

Related:
alwaysThinkingEnabled is already readable from settings.json, but effort level is session-only state and not persisted anywhere accessible to external scripts.