E2E case compatibility pointer

The numbered full-stack behavior-case source of truth moved to /error-cases.

Each CASE-###-slug/ directory owns an at-a-glance USER-JOURNEY.md, its detailed human record, machine-readable manifest, and executable full-stack journey. Both incident regressions and proactive expected-behavior flows live there, with either the standard local/production profile or a live production external-OAuth profile. This compatibility directory remains only so historical links do not break.