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/agents/implementer.md

@@ -2,7 +2,7 @@
 name: implementer
 description: Carries out a tightly scoped implementation task. Reads the task extract handed by the controller, implements, builds, tests, reports a structured status with the diff. NOT for architecture decisions, NOT for self-curated scope; this agent executes a plan that has already been made.
 tools: Read, Edit, Write, Bash, Glob, Grep
-model: opus
+model: sonnet
 effort: high
 ---