evals: prompts leak the expected answer; step6 cases have no scaffold #389

Evals tell the subject the expected answer. evals/deep-audit-step6-zero-fixes-skip/prompt.md:15-16,55-56 says the code has 'no bugs', which the grader expects (graders/step6-skipped-zero-fixes.md:6); single-local-fix-skip/prompt.md:15-16 gives away the step-6 trigger. All six step6 cases lack scaffold.sh, so the subject builds the code it audits. Fix: move fixture setup into scaffold.sh and add a check in tests/evals/test-eval-cases.sh that fails on these tells (a prototype flagged exactly those four lines). Open: whether scaffold.sh can git init here; one paid --keep-temp run settles it.

From the 2026-10-03 pstack-claude adoption audit (docs/research/pstack-claude-audit-2026-10-03.md, PR #385). Constraints: bash 3.2 and Python 3.9, failing test first (Rule 4), no hardcoded home paths, stage by name.