ci: build multi-platform Docker images (amd64 + arm64)

Image only had linux/amd64, failing on Apple Silicon Macs with
"no matching manifest for linux/arm64/v8". Added QEMU + buildx
multi-platform support.

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

.github/workflows/release.yml

@@ -102,6 +102,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - uses: docker/setup-qemu-action@v3
+
       - uses: docker/setup-buildx-action@v3
 
       - uses: docker/login-action@v3
@@ -114,6 +116,7 @@ jobs:
         with:
           context: .
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: |
             ghcr.io/0xmassi/webclaw:latest
             ghcr.io/0xmassi/webclaw:${{ github.ref_name }}