alternative backend support

luria is designed around a markdown knowledge garden as first class citizen, but there's no reason a user shouldn't be able to e.g. store their content in sqlite or whatever.

The issue names no files, tests, or entry points. Start by tracing how Luria currently stores and reads its markdown knowledge garden, then determine the backend boundary and behavior an alternative such as SQLite would require; done would mean a concrete design with agreed storage semantics and coverage for the new backend.

Written by the indexing model from the issue text.