[ai] Delegate impl phases to opus.

.claude/ai-workflow-adapter.md

@@ -14,6 +14,14 @@ This file adapts harness mechanics and removes unnecessary text normalization.
   rely on the parent conversation being inherited.
 - Tell every leaf Agent not to delegate and never to commit. Preserve the
   shared workflow's single-writer and one-stateful-performer constraints.
+- Override the shared inherit-the-parent-model rule for the mechanical
+  leaves: spawn every Phase 4 implementation-unit leaf, every Phase 6b
+  review-fix leaf, and every test-loop impl-fix leaf with `model: "opus"`
+  (Opus 4.8) on the Agent call. Do not pass a reasoning field — the Agent
+  tool has none, and effort is inherited unchanged, so these leaves keep
+  the parent's maximum reasoning level. Every other phase — context, plan,
+  assessment, review, build, test author — and the performer itself
+  inherit the parent model as the shared workflow says.
 - A foreground Agent call may replace Codex-specific polling. Treat its short
   reply as notification and validate the required files and repository state.
   When Claude exposes a resumable agent id and more work is needed from that