[FEATURE] Spinner elapsed time should show total time since the prompt (or expose turn elapsed to the statusline)

Feature request

Show total elapsed time since the user submitted their prompt in the working spinner (for example "Brewed for 54s"), or expose a turn-elapsed field to the custom statusline.

Current behavior

The spinner's seconds counter resets per output segment / API request. During a long multi-step turn (many tool calls, background tasks, subagents), the display reads as "54s" when the turn has actually been running for many minutes. The number answers "how long has this segment streamed" when the question a user is asking is "how long have I been waiting since I hit enter."

What I looked for first

• No setting in settings.json, env vars, or /config controls this display.
• The custom statusline input includes cost.total_duration_ms and cost.total_api_duration_ms, but both are SESSION totals; there is no field for time since the current user prompt, so a custom statusline cannot show it either.

Desired

Either of:

1. The spinner shows cumulative elapsed time for the whole turn (time since the user's message that started it), or shows both ("12m turn / 54s this step").
2. A turn_duration_ms (or prompt_submitted_at) field in the statusline stdin JSON, so users can render it themselves with refreshInterval.

Context: observed during long autonomous sessions where a single turn spans many minutes of builds and background tasks; the per-segment number makes it hard to tell how long the work has actually been running.