eval: track nightly full-eval green streak (3 consecutive days)

[SebAustin]

---

title: "eval: track nightly full-eval green streak (3 consecutive days)"
labels: [eval, w2]
assignees: [SebAustin]

Problem

W1 acceptance requires the nightly workflow to stay green for three
consecutive days. Today we only have a pass/fail badge with no streak
counter or regression alert.

Acceptance criteria

• Nightly workflow uploads summary.json artifact and posts a job
  summary with mean_judge_score, n_pass, and total_cost_usd.
• Document the green-streak requirement in README (Eval targets).
• Optional: GitHub issue or discussion opened automatically when nightly
  fails twice in a row.

References

• .github/workflows/eval-nightly.yml
• prompts/05_evals_full_run.md