fix: space/index/add uses correct reader (#44)

I tried writing a more sensible test that would actually hit this, but I
don't think there's currently a good way to do that. I could force it,
but it's not worth the effort—what we really need is to rework
`go-ucanto`, and that's not a now project. Until then, this makes the
capability work as expected.

Author: Peeja

capabilities/space/index/add.go

@@ -52,7 +52,7 @@ var AddOkReader = schema.Struct[AddOk](AddOkType(), nil, types.Converters...)
 var Add = validator.NewCapability(
 	AddAbility,
 	schema.DIDString(),
-	schema.Struct[AddCaveats](nil, nil, types.Converters...),
+	AddCaveatsReader,
 	func(claimed, delegated ucan.Capability[AddCaveats]) failure.Failure {
 		if fail := equalWith(claimed, delegated); fail != nil {
 			return fail