feat: implement issue #1900 — [Phase 3] Refresh the Opus 4.8 fallback-chain hops to Opus 5.5 in engine.sh

Merged
DO
10d ago
+42-35

Problem

[Phase 3] Refresh the Opus 4.8 fallback-chain hops to Opus 5.5 in engine.sh

From the issue: As a review-engine maintainer, I want the last Claude fallback hop that still falls back to Opus 4.8 from a non-Opus model to use Opus 5.5 instead, so that the action tier's last resort is the cheaper, proven model now that the deep-tier canary has passed (#1899).

Risk

Low — changes automation shell logic under scripts/, covered by shellcheck (--severity=warning) and the bats suite.

Test plan

Tests added/updated: tests/dev-lead/unit/test_engine_chain.bats,tests/dev-lead/unit/test_engine_models.bats tests/dev-lead/unit/test_engine_new_models.bats. Verification: bash scripts/dev-lead-lint.sh (shellcheck --severity=warning) ran pre-commit; the bats suite runs in CI.

Rollback

Revert this PR. No non-revertible side effects (no tags, migrations, or external state).

Activation

Inert on merge. The action-tier default only takes effect once a release channel is cut onto this commit: dev-lead runs it from its dev-lead/v139-* channel tags and pr-review from pr-review/v1-*. Until a channel moves, every agent keeps the previous chain.

Monitoring

This PR's Lint (shellcheck) and bats checks show pass/fail. Behavioural monitoring starts after the channel cut, not after the merge: watch the writer's exit-2 (all hops rate-limited) rate, because the last hop now shares Opus 5.5's rate-limit bucket with the deep, audit and single primaries.

Closes #1900