refactor(review): remove the unused ratchet_offer setting #11973

Parent: #11129

Blocked by: #11969

Slice 4 of 4 of #11129. Slice 2 removed /review:audit-enforceability, the only reader of review's
ratchet_offer; /corrections:codify reads its own ratchet_offer (plugins/corrections/schemas/corrections.schema.json).

Decisions that bind this slice

• From feat(corrections): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them #11121: the removed skill goes with its schema keys, config entries and README rows.
• Split by hand by the Issue Triage session after the issue lane's scan refused each run's patch
  as too large. No owner decision is claimed for this split.

Task list

• Remove ratchet_offer from plugins/review/schemas/review.schema.json, .claude-plugin/plugin.json
  (userConfig), README.md, reference/config.md and docs/contracts/config-cascade/README.md's review row.
• Move /review:setup's cases to downstream_probe: skills/setup/SKILL.md, skills/setup/evals/evals.json,
  skills/setup/scripts/setup-apply.test.sh, the quoted --check output in skills/fanout/evals/evals.json
  (lines 581, 593, 606), and plugins/review/evals/su-accepted-default-records-default/
  and su-chosen-default-value-records-chosen/ (rewrite to downstream_probe, or delete a case that only
  ratchet_offer exercised).

Done when

• rg -n 'ratchet_offer' plugins/review docs/contracts/config-cascade --glob '!**/CHANGELOG.md' prints nothing.
• bash scripts/run-test-file.sh plugins/review/skills/setup/scripts/setup-apply.test.sh passes.
• bash plugins/skill-quality/scripts/check-skill.sh plugins/review/skills exits 0, and
  python3 plugins/evals/skills/validate/scripts/validate-cases.py plugins/review/evals exits 0.
• scripts/check-changelog-fragments.sh passes with a review fragment.

Out of scope

• downstream_probe behavior; corrections' own ratchet_offer.