Adopt selected Hugging Face improvements (FP-166)

[sambokai]

Implements FP-166.

Summary:

• Add Hugging Face credential discovery for HF_TOKEN, HUGGING_FACE_HUB_TOKEN, and the documented Hugging Face CLI token paths while preserving configured/token-account HF_TOKEN precedence.
• Keep billing-first reported spend authoritative and add a 12-hour process-local identity cache with fail-soft enrichment.
• Add provider-scoped authentication, billing permission, rate-limit, and retry diagnostics.
• Add focused coverage and update provider documentation.

Validation:

• make check
• Hugging Face-focused suite: 41 tests across QuickJS and JavaScriptCore
• make test: 1,003 selections in 84 groups, 0 failures, 0 retries, 0 timeouts
• node --check and git diff --check

No deviations from the FP-166 implementation scope. steipete#3399 billing semantics remain intact; usage-v2, ZeroGPU, synthetic gauges, and shared-runtime changes are excluded.