HaikuRAG resolves `config=None` to the global config for everything
else; the hook list read the raw argument, so a client constructed
without an explicit config never loaded its configured hooks.
Tests follow the config and context-expansion APIs: `get_config()`
replaces the removed `Config` instance, and the expansion test does
its own `resolve_refs_grouped` / `get_items_in_ranges` fetch now that
`expand_with_items` takes the items rather than fetching them.
