test(W7h): delete the codex-offering prompt builder left unused by the retirement

Dead-code follow-up to the codex-offered-* retirement: buildCodexOfferingPrompt
(test/helpers/codex-offering-fixture.ts) had no caller besides those cases.
Deletes the helper, its free test (test/codex-offering-fixture.test.ts), the
two captured public fixtures it replayed, its scripts/free-test-durations.json
entry, and the helper/fixture paths still listed as touchfiles of six plan
review E2Es in test/helpers/touchfiles-data.ts.

Retirement card:
- test: codex-offering-fixture.test.ts (pinned the builder's prompt text and
  replayed captured timeout/CDD transcripts against it).
- detects: changes to a prompt that no paid case sends any more.
- non_test_callers: none (grep -rn codex-offering test scripts is empty).
- replacement: none needed; the behavior it supported is now the free probe
  check in gen-skill-docs.test.ts.
- validation: bun test test/touchfiles.test.ts test/touchfiles-map-diff.test.ts
  test/paid-shards.test.ts test/e2e-tier-alignment.test.ts; bun run typecheck.

scripts/free-test-durations.json

@@ -312,7 +312,6 @@
     "test/codex-generation-model.test.ts": 60,
     "test/codex-hardening.test.ts": 1675,
     "test/codex-model-probe.test.ts": 263,
-    "test/codex-offering-fixture.test.ts": 36,
     "test/codex-resume-flag-semantics.test.ts": 14,
     "test/codex-session-lifecycle.test.ts": 12041,
     "test/codex-under-codex-detection.test.ts": 35,