define a minimal interface to the reasoner

As an initial pass, we could define a Reasoner API in terms of only horned-owl, so that we don't expose internals of Whelk, which may change.

Start by reviewing the repository's current reasoner implementation and how it uses horned-owl; the issue does not name specific files or tests. Clarify the minimal public Reasoner API and the internals it should hide, then document the agreed interface and verify that existing reasoner behavior remains accessible through it.

Written by the indexing model from the issue text.