fix: add rustfmt to Docker image for formatting checks

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

docker/Dockerfile

@@ -9,8 +9,8 @@
 
 FROM rust:1.90-bookworm AS base
 
-# Clippy is needed at runtime for acceptance gates (cargo clippy)
-RUN rustup component add clippy
+# Clippy and rustfmt are needed at runtime for acceptance gates
+RUN rustup component add clippy rustfmt
 
 # ── System deps ──────────────────────────────────────────────────────
 RUN apt-get update && apt-get install -y --no-install-recommends \