Expose "extra usage" balance and spend in statusline JSON input

Feature request

Claude.ai Max subscribers can enable "extra usage" — a prepaid pool (with a monthly spend limit) that acts as overage when 5h/7d rate limits are hit. This is visible on claude.ai Settings → Usage:

• Monthly spend limit (e.g. $50)
• Amount spent this period (e.g. $8.37 — 17% used)
• Current balance (e.g. $11.62)
• Reset date (e.g. "Resets May 1")
• On/off toggle state
• Auto-reload state

None of this is exposed in the JSON input sent to statusline scripts. rate_limits.five_hour and rate_limits.seven_day are there, but the extra-usage pool is invisible from Claude Code.

Use case

I'd like to see in the statusline how much extra usage I've spent this month (or how much of the spend limit remains) without having to switch to the browser. This is especially relevant for users who work long sessions and regularly hit rate limits — the pool exists to prevent interruptions, but budgeting against it is hard when the balance isn't visible.

Proposed fields

{
  "extra_usage": {
    "enabled": true,
    "spent_usd": 8.37,
    "monthly_limit_usd": 50.00,
    "used_percentage": 16.74,
    "balance_usd": 11.62,
    "resets_at": "2026-05-01T00:00:00Z",
    "auto_reload_enabled": false
  }
}

(Names are suggestions — any stable schema works.)

Alternatives considered

• Polling the claude.ai dashboard — requires session cookies, no public API, fragile.
• Manual check in browser — defeats the purpose of statusline awareness.