Mem-Client: Validation Evidence

Mem-client behavior changes need validation evidence.

Scope:
- Enforce only when changes touch `domains/frontend/mem-client/src/**`, `domains/frontend/mem-client/e2e/**`, or mem-client runtime config.
- Do not enforce for pure docs/changelog/version-only updates.

Rules:
1) For behavior-changing mem-client work, PR description should include a short `Validation` section.
2) Validation should include exact commands or concrete manual verification steps.
3) Include pass/fail outcomes for the listed checks.

Review behavior:
- Return NOT APPROVED only when behavior-changing mem-client work has no credible validation evidence.
- Do not block clearly non-behavioral changes.