HTTP client that calls GET /pass-rate?skill=X&window=Y on the brain pod.
Caches *float64 results (including nil) per-skill for the configured TTL
(default 60s). On non-200 or network error returns (nil, err) so the
upstream router can fall through to default-to-local.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
