docs(evals): add model-judge calibration notes

[aryeko]

Summary

• add curated technical-design pointwise model-judge calibration notes
• state deterministic verdicts remain authoritative and model-judge evidence is manual/advisory
• document that expected-bad false-pass calibration is deferred to Phase C4

Verification

• pnpm install --frozen-lockfile
• pnpm eval:doctor
• pnpm eval:list
• pnpm eval:validate-fixtures
• pnpm eval:judge:doctor
• pnpm eval:judge:list
• pnpm eval:judge:validate-fixtures
• pnpm check
• git diff --check

No provider-backed evals were run. Raw evals/results bundles remain local/ignored.