The git half works here exactly as it does anywhere: rig stack init fuses the
repos, a commit spans them, send and push take work back out.
The build half is not implemented for npm yet. rig stack wire reports that
it skipped, rather than pretending. npm has more than one way to do this — an
overrides block, a workspaces array, a linked dependency — and picking one
without a real repository to check it against is how you end up with a
generator nobody can use.
Until then, wire it by hand with whichever of those suits your setup. The manifest and the layout below are the parts that are already right.
If you want this, the implementation goes in core/ecosystem/node as a
LocalOverlay method — the same one the .NET and Go adapters implement — and
this directory becomes its test.