getUserByUsername emitted the FULL username as the emailDomain span attribute whenever the username contained no "@" (audit finding B6), violating the PII-free telemetry rule (ADR-017 §7). Emit only a boolean hasAtSign in both the production repository and its mock; regression tests pin that no username-derived string reaches span attributes. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>