refactor(redact): reconcile stale 7-layer/8-layer vocabulary tree-wide

The sb_secret_ addition brought the always-on/opt-in regex pipeline from 7 to 8 layers, but only redact.go, docs/security-and-privacy.md, and CLAUDE.md were updated at the time. ~25 sibling references across the tree still said "7-layer" for what is now the 8-layer regex pipeline, and a couple said "8-layer" for what OPF now makes the 9th, network-backed layer — leaving the tree internally inconsistent about layer counts.

Reconcile every reference to the true counts (8 always-on/opt-in regex layers; OPF as the opt-in 9th, network-backed layer), and rename the now-misleadingly-named apply7LayerToBlobs (which applies all 8) to applyRegexLayersToBlobs so the identifier no longer encodes a count that drifts every time a layer is added.