Per Anton: project agents should run on Opus 4.8, not just the reviewers.
Previously only deep-code-reviewer + auto-code-reviewer were on `opus`;
workers (backend/frontend/database/devops/qa-tester/code-reviewer + auto-*)
were on `sonnet` and tech-analyst on `haiku`. Bump all of them to `opus`
so implementation quality matches the reviewer tier (esp. under ultracode).
Guard files untouched: _autonomous_pickup.md (no model field),
auto-code-reviewer.md (already opus).
