Add eval system Phase 5 — debug UI ...

Self-contained SPA at /debug/eval (route already wired in 8e0eed6).
Single index.html in the existing debug/ style — vanilla JS, embedded
CSS, no framework, no build step. Four tabs:

- Sessions — filterable table (profile / status / limit), eval status
  pill, headline avg scores, click-through to detail
- Detail — session metadata + every stored eval run, axes laid out as
  axis × expert grids with inline averages, expert comments, button to
  re-evaluate this single session
- Stats — weekly per-axis means table, optional complexity-bucket split
- Run — form to trigger any scope (unevaluated / single / all), live
  status panel polling /eval/run/{id} every 2.5s, run history with
  click-to-attach

Hash routing: #detail/<session_id> deep-links to a session.

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