feat(models): move the implementer to opus/high

Owner-ratified 2026-07-20: authored code is the pipeline's primary
output, so plan execution takes the judgment model — but keeps the
in-loop effort tier (high, not xhigh): the implementer is the
most-dispatched per-task role and volume x wall-clock still governs
its thinking budget. Applies to the frontmatter pin and the three
implement-loop dispatches (impl, impl-fix-spec, impl-fix-qual).

The compiler-driven-edit arm deliberately keeps its implementer
dispatch at sonnet/high: compiler-enumerated propagation is
mechanical, and the arm's policy header pins its own tier.

implement/workflows/compiler-driven-edit.js

@@ -43,7 +43,10 @@
 // session-model inherit (which could route to an unintended tier, e.g. fable —
 // reserved for the four ratified frontmatter gates, never for workflow stages;
 // agent-template § model rule 1). Both phases here are mechanical
-// (compiler-enumerated propagation, build/suite re-run), so both run sonnet.
+// (compiler-enumerated propagation, build/suite re-run), so both run sonnet —
+// including the implementer agentType: its opus frontmatter tier is for plan
+// execution, not compiler-enumerated propagation (agent-template § model
+// rule 3), so this arm pins sonnet explicitly.
 // Effort policy mirrors it: every call pins an explicit effort — the edit
 // (real code work) runs high, the build/suite verify (schema-bound check)
 // runs medium. See docs/agent-template.md § effort.