In order to create a table that inherits from core.companies, devuser must be the owner of core.companies. Also, devuser must have usage privileges on the schema "core".
I couldn't find this in the docs. I determined the answer through testing.
You might be able to work around this by having coreuser create child tables in the dev schema, then either grant privileges to devuser or change ownership to devuser. I think devuser will need some privileges on core.companies in order to update data, but I could be wrong.