Feature request: Include billing period usage % in statusline JSON input

Feature Request

Is your feature request related to a problem?

I use a custom statusline script (via settings.json statusLine.command) to display session info. I'd love to also show how much of my billing period usage I've consumed, but this data isn't included in the JSON piped to the statusline command.

Describe the solution you'd like

Include billing/usage period information in the JSON input passed to custom statusline commands, e.g.:

{
  "billing": {
    "period_start": "2026-02-01T00:00:00Z",
    "period_end": "2026-03-01T00:00:00Z",
    "usage_percentage": 42.5
  }
}

This would let users display something like usage: 42% in their statusline, helping them pace their usage through the billing period.

Describe alternatives you've considered

• Scraping the billing page or API separately — fragile and requires extra auth
• Estimating from cost data — inaccurate since it doesn't account for the plan's actual limit

Additional context

The statusline JSON input already includes cost.total_cost_usd for session cost, so adding billing period context would be a natural extension.