Expose effort level in status line JSON input

Problem

The current effort level is not persistently visible anywhere. When set via /effort (session-only), it briefly flashes in the notification bar but then disappears. There's also no way for custom status line scripts to display it, since the status line JSON input doesn't include effort level.

Related to #9488 which tracks exposing thinking mode / reasoning effort in status line JSON.

Proposed solutions

Either (or both):

1. Built-in status bar option — allow effort level to be shown persistently in the default status bar (similar to how model name is always visible)

1. Expose in status line JSON — add an effortLevel field to the status line JSON input so custom scripts can display it. Should reflect the effective effort (session override > env var > settings.json default).

{
  "model": {
    "id": "claude-opus-4-6",
    "display_name": "Opus 4.6 (1M context)",
    "effort": "max"
  }
}