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

This reverts commit cdbc2d0. The bump was a judgment call without
evidence, and the planner design intent argues the other way: the plan
Iron Law is "EVERY STEP THAT CHANGES CODE INCLUDES THE CODE" and tasks
are sized so a subagent executes one "without judgement calls outside
its remit" (planner/SKILL.md) — the planner exists precisely to remove
design latitude from the implementer, which points at sonnet, not opus.
opus is only justified to the degree real plans leave slack (RED-step
invention, tree reconciliation, review repair), and that slack is
measurable, not felt. Restore sonnet/high; a future bump should be
grounded in the loop metrics (spec re-loop count, quality
changes_requested rate, end-verify BLOCKED rate) via postmortem, not a
gut call. The four fable gates (b591e75) are untouched.

implement/workflows/compiler-driven-edit.js

@@ -43,10 +43,7 @@
 // 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 —
-// 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.
+// (compiler-enumerated propagation, build/suite re-run), so both run sonnet.
 // 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.