"type:" as a label prefix

reading sessions already do this in the data apparently as `{"type": "reading_session"}`. This is a great idea and naturally extends meta-attributes: as labels. I was basically doing this already with UID.

anyway. yeah. gh-store should "handle" object type. lol, could even integrate pydantic. or maybe I already did?

Start by tracing how reading sessions currently encode {"type": "reading_session"} and how UID is used. Clarify the intended label representation and whether Pydantic is in scope; done means gh-store consistently handles object types as label metadata.

Written by the indexing model from the issue text.