Expose fast mode status in statusLine JSON input

Feature Request

The statusLine JSON input currently does not include whether fast mode (/fast) is enabled. This makes it impossible to display the current fast mode state in a custom status line.

Context

The statusLine JSON exposes useful session metadata (model, context window, cost, etc.), but is missing two key operational states:

• Fast mode (this issue) — whether /fast is toggled on
• Effort level (already requested in #24758)

For users who frequently toggle between fast and normal mode, there's no visual indicator of the current state beyond remembering what you last set.

Proposed Change

Add a field to the statusLine JSON input:

{
  "fast_mode": true
}

Use Case

Custom status lines that show the full operational context, e.g.:

Opus 4.6 | fast | ctx: 28% | 10k tokens

This is especially useful for Max plan users who toggle fast mode depending on the task complexity.

Related Issues

• #24758 — Expose effort level in statusline input JSON
• #26108 — Expose rate limit data in statusline JSON
• #25891 — Expose /usage data in statusLine JSON API