feat: per-provider credit balance reporting in observability hooks

[stackbilt-admin]

Summary

llm-providers should expose per-provider credit/balance visibility through its existing observability hooks surface. Currently AEGIS has no real billing data — SENTINEL alerts about negative credits are heuristic guesses with no ground truth.

Motivation

• AEGIS SENTINEL flagged "AI credit balance is negative (-50)" with no actual billing data backing it
• Each provider exposes usage/billing APIs that llm-providers already has credentials for
• This is a natural extension of the existing circuit breaker + observability hooks — not a new concept

Providers & APIs

Provider	Endpoint	Notes
Anthropic	GET /v1/usage	Token counts by model
Groq	Usage API	Generous free tier
Workers AI	CF API	Free tier — usage metrics via CF dashboard API
Brave	Response headers	Monthly quota in X-RateLimit-Remaining

Proposed Surface

• getProviderBalance(provider?) — returns current credit/quota state per provider
• Wire into existing health/stats reporting so consumers (AEGIS, dashboards) can query balance without custom integrations
• Optionally emit low-balance warnings through the existing hook system

Acceptance Criteria

• At least Anthropic + Groq balance reporting implemented
• Exposed through observability hooks (same pattern as existing metrics)
• AEGIS can consume via service binding or direct import