Add remaining credit balance field to statusLine JSON payload

The statusLine JSON payload currently exposes cost.total_cost_usd (estimated session spend) and rate_limits.five_hour/seven_day (Pro/Max subscription usage windows), but there's no field for accounts on usage-based/credits billing to see their remaining prepaid credit balance.

It would be useful to expose something like account.remaining_credit_usd (or similar) in the statusLine payload so API-credit users can build a statusline that shows how much balance is left, the same way subscription users can already see rate_limits.* usage.

Currently the only way to check remaining credits is the Anthropic Console billing page, which isn't convenient for an at-a-glance statusline.