Fix ImageExists to use skopeo instead of podman manifest inspect

podman manifest inspect only works for multi-arch manifest lists,
returning exit code 125 for regular single-arch images. Switch to
skopeo inspect which works for both.

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

internal/runtime/podman.go

@@ -199,15 +199,16 @@ func (p *Podman) Push(ctx context.Context, image string) error {
 }
 
 // ImageExists checks whether an image tag exists in a remote registry.
+// Uses skopeo inspect which works for both regular images and multi-arch
+// manifests, unlike podman manifest inspect which only handles manifests.
 func (p *Podman) ImageExists(ctx context.Context, image string) (bool, error) {
-	cmd := exec.CommandContext(ctx, p.command(), "manifest", "inspect", "docker://"+image) //nolint:gosec // args built programmatically
+	cmd := exec.CommandContext(ctx, "skopeo", "inspect", "--tls-verify=false", "docker://"+image) //nolint:gosec // args built programmatically
 	if err := cmd.Run(); err != nil {
-		// Exit code 1 means the manifest was not found.
 		var exitErr *exec.ExitError
-		if ok := errors.As(err, &exitErr); ok && exitErr.ExitCode() == 1 {
+		if ok := errors.As(err, &exitErr); ok && exitErr.ExitCode() != 0 {
 			return false, nil
 		}
-		return false, fmt.Errorf("podman manifest inspect %q: %w", image, err)
+		return false, fmt.Errorf("skopeo inspect %q: %w", image, err)
 	}
 	return true, nil
 }