Rewrite eval_system.md as user guide; preserve original spec as eval_system_design.md ...

- docs/eval_system.md: replaced stale spec with current user guide
  covering UI tabs, CLI, rubric, experts, versioning, API
- docs/eval_system_design.md: preserved original design spec for
  reference

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>