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>

Add eval system Phase 1 — message feedback signal ...

Spec at docs/eval_system.md describes the full LLM-as-judge plan;
this commit lands only the in-app feedback layer:
- debug/eval/ Python package with EvalDB (asyncpg) and FastAPI router
  exposing /eval/feedback (set / clear / list)
- message_feedback postgres table keyed by (session_id, message_index)
- thumbs up / down on each completed assistant block in the webclient,
  optimistic update with rollback on failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>