Expose fast mode status in statusLine JSON

Summary

The statusLine JSON input (used by custom statusline-command scripts) does not include any field indicating whether fast mode (/fast) is active. This makes it impossible to show a fast mode indicator in a custom status line.

Current behavior

When fast mode is toggled via /fast:

• The built-in ↯ icon appears next to the prompt
• No field changes in the statusLine JSON — model.id, model.display_name, output_style.name, and all other fields remain identical

Verified empirically on v2.1.83 by dumping the full statusLine JSON with fast mode off and on — the payloads are identical except for cost/token counters.

Requested change

Add a fast_mode boolean (or similar) to the statusLine JSON schema:

{
  "model": { "id": "claude-opus-4-6[1m]", "display_name": "Opus 4.6 (1M context)" },
  "fast_mode": true,
  ...
}

This would allow custom statusline scripts to render a visual indicator, e.g.:

fast_mode=$(echo "$input" | jq -r '.fast_mode // empty')
if [ "$fast_mode" = "true" ]; then
  parts+=("\033[0;34mFAST\033[0m")
fi

Why

Users who customize their status line (via statusline-command in settings) lose the fast mode indicator because the ↯ icon is rendered separately by the CLI. The status bar is the natural place to surface mode state alongside model name, context usage, and cost.