ci: use pre-built binaries for Docker instead of QEMU cross-compilation

QEMU arm64 Rust builds took 60+ min and timed out in CI. Now the Docker
job downloads the pre-built release binaries and packages them directly.

- Dockerfile.ci: slim image for CI (downloads pre-built binaries)
- Dockerfile: full source build for local dev (unchanged build stage)
- Both use ubuntu:24.04 (GLIBC 2.39 matches CI build environment)
- Multi-arch manifest combines amd64 + arm64 images

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

Dockerfile.ci

@@ -0,0 +1,16 @@
+# Slim runtime image — uses pre-built binaries from the release.
+# The full Dockerfile (multi-stage Rust build) is for local development.
+# CI uses this to avoid 60+ min QEMU cross-compilation.
+ARG BINARY_DIR=binaries
+
+FROM ubuntu:24.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+ARG BINARY_DIR
+COPY ${BINARY_DIR}/webclaw /usr/local/bin/webclaw
+COPY ${BINARY_DIR}/webclaw-mcp /usr/local/bin/webclaw-mcp
+
+CMD ["webclaw"]