Improve platform detection and add multi-arch run test

- Updated get_test_platform() to detect runtime architecture in CI
  - CI tests use native platform (linux/amd64 on x86_64, linux/arm64 on aarch64)
  - Local development always uses linux/amd64 for consistency

- Added test_multi_arch_build_and_run() test
  - Builds multi-arch image and pushes to ttl.sh
  - Verifies the image runs correctly on the current architecture
  - Gracefully handles missing toolchain scenarios

- Updated CI integration test to verify multi-arch images can run
  - Builds for both linux/amd64 and linux/arm64
  - Runs the image to ensure Docker selects the correct architecture

This ensures we properly test both single and multi-arch scenarios.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -162,10 +162,15 @@ jobs:
       run: |
         cd example/hello-krust
         krust build --no-push --image local.test/hello:latest ./
-    - name: Test multi-arch build
+    - name: Test multi-arch build and run
       run: |
         cd example/hello-krust
-        krust build --no-push --platform linux/amd64,linux/arm64 --image local.test/multiarch:latest ./
+        # Build for both platforms and push
+        export KRUST_REPO=ttl.sh/${{ github.run_id }}-multiarch
+        IMAGE_REF=$(krust build --platform linux/amd64,linux/arm64 ./)
+        echo "Built multi-arch image: $IMAGE_REF"
+        # Run the image - should automatically select the right architecture
+        docker run --rm $IMAGE_REF
 
   # Takes too long to run on CI, so it's commented out for now.
   # coverage: