Expose /fast mode and /effort level in statusline JSON data

Follows up on #24279 (closed by stale bot).

There's no way to detect /fast mode or /effort level from a statusline script. When either is toggled, the statusline JSON stays identical — model.id, model.display_name, and all other fields are unchanged.

It'd be great to have something like:

{
  "fast_mode": true,
  "effort": "high"
}

...in the statusline JSON so custom scripts can surface these. Right now there's no visual indicator of either setting outside the chat output itself, which makes it easy to forget what mode you're in.