The addon chart published by hack/addons/syn_addon_package.go does not generate a proper helm chart with a Chart.yaml in it.

Why not use the existing vela cli ability to publish addons instea.
It properly covers the generation of the Chart.yaml file as shown with vela addon package that is using the MakeChartCompatible routine from
https://github.com/kubevela/kubevela/blob/e109d4e525dfc253e861c82901d3947688555d7c/pkg/addon/utils.go#L367

Note: In my case I am trying to install some vela addons (ex:fluxcd) automatically after installing vela-core using helmfile.
I can succesfully install fluxcd manually using
vela addon enable fluxcd --dry-run --version 2.3.4 | kubectl apply -f -
but ultimately I would prefer to also install the addon using helmfile and a hook for vela --dry-run