Add clippy to Docker image

Acceptance gates run cargo clippy but the component wasn't installed
in the build stage. Agents were doing real work then failing every
gate check because clippy wasn't available.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/Dockerfile

@@ -9,6 +9,9 @@
 
 FROM rust:1.90-bookworm AS base
 
+# Clippy is needed at runtime for acceptance gates (cargo clippy)
+RUN rustup component add clippy
+
 # ── System deps ──────────────────────────────────────────────────────
 RUN apt-get update && apt-get install -y --no-install-recommends \
         git \