Machine-readable CEO operational credit health.

Designed for agent consumption: an agent can poll this endpoint before negotiating an expensive deal to avoid wasted round trips when the credit pool is exhausted. The CEO dashboard also reads this via a same-origin proxy.

Public surface intentionally minimal -- exposes health band, utilization percentage, and remaining USD headroom but NEVER the raw PHP credit limit. The raw limit is operationally sensitive (gives an attacker the exact spending ceiling); the available_usd figure is the only number an external agent legitimately needs.

Distinct from the older /credit-health endpoint (zero-state scaffold from Phase 36, never wired to ap2_executions). This endpoint reads live spend data. /credit-health is preserved for backward compatibility with the boardroom Credit Tank widget.

No authentication required -- safe-by-design response surface.