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/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: sonnet
+model: opus
 effort: high
 ---