refactor(session-flow): remove retro's codify mode #11970

Parent: #11129

Blocked by: #11962

Slice 3 of 4 of #11129. It removes retro's codify [reviews] mode, now covered by
/corrections:codify (its reviews action reads PR review comments, the author's own after merge
included).

Decisions that bind this slice

• feat\(corrections\): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them - part 2 of 2 #11129 is part 2 of feat(corrections): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them #11121, split at intake under Q93 (the intake split comment on feat(corrections): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them #11121); feat\(corrections\): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them - part 1 of 2 #11128 (part 1) gave
  /corrections:codify the PR-review-comment source and ported this mode's eval cases.
• From feat(corrections): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them #11121: retro's codify mode is removed with its probes, evals, schema keys, config entries and
  README rows, and every reference moves to /corrections:codify.
• 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 (from #11129, lines 1-2, scoped to this slice)

• Delete plugins/session-flow/skills/retro/context/codify.md, scripts/fetch-review-comments.sh,
  scripts/fetch-review-comments.test.sh, evals/fixtures/review-comments.jsonl, and
  plugins/session-flow/evals/re-codify-routes-check-asks-first/.
• Remove the mode from retro's SKILL.md (argument hint, mode list, and the fetch-review-comments.sh
  entry in allowed-tools) and its cases from
  evals/evals.json; repoint reference/ecosystem-improvement-catalog.md.
• Remove or repoint the mode in session-flow's .claude-plugin/plugin.json, README.md,
  prerequisites.json, reference/config.md and schemas/session-flow.schema.json (drop encode_policy
  and review_mining_prs; corrections owns review_mining_prs now), in
  skills/setup/SKILL.md (lines 80-81, 176, 185) and skills/setup/scripts/setup-apply.test.sh (the
  cases at lines 257-295), delete skills/retro/evals/fixtures/session-flow.yaml (only the codify case at
  evals/evals.json:105 uses it), and repoint skills/running-retro (SKILL.md, context/checkpoint.md,
  evals/evals.json), skills/audit-sessions (SKILL.md, evals/evals.json, scripts/sweep.py),
  skills/audit-friction/reference/classes.md, skills/workflow/context/wrap-up.md, and
  evals/rr-routes-never-codifies/ (case.yaml, samples).
• Repoint the session-flow rows of docs/contracts/config-cascade/README.md, and any docs/catalog.md
  or docs/skill-cheat-sheet.md text that names the mode.

Done when

• retro's argument hint has no codify mode.
• rg -n 'review:audit-enforceability|retro codify' plugins/session-flow --glob '!**/CHANGELOG.md' --glob '!.changes/**' prints nothing,
  and rg -n 'retro.? codify' docs/contracts/config-cascade docs/catalog.md docs/skill-cheat-sheet.md prints nothing
  (the review:audit-enforceability lines in those docs are slice 2's).
• bash scripts/run-test-file.sh plugins/session-flow/skills/setup/scripts/setup-apply.test.sh passes, and
  rg -n 'encode_policy|review_mining_prs' plugins/session-flow docs/contracts/config-cascade --glob '!**/CHANGELOG.md' prints nothing.
• bash plugins/skill-quality/scripts/check-skill.sh plugins/session-flow/skills exits 0.
• scripts/check-changelog-fragments.sh passes with a session-flow fragment.

Out of scope

• /review:audit-enforceability and any file under plugins/review (slices 2 and 4).
• audit-sessions style; chat and meeting sources (out of scope in feat(corrections): absorb retro codify and /review:audit-enforceability into /corrections:codify and remove them #11121).