k8s-pvc-growth-forecast — days-to-full for every PVC, not another 95% page

Forecast when every PersistentVolumeClaim in a cluster runs out of space: pull kubelet volume stats from Prometheus, fit a growth rate per PVC over a lookback window, and report days-to-full so a disk that fills in nine days becomes a ticket instead of a 3am page. Exits non-zero in CI when any volume crosses the warning horizon.