Keyword search raises `ZeroDivisionError` when the index is empty

[Aburke225]

KeywordSearcher.search() crashes with ZeroDivisionError (inside rank-bm25's IDF computation) when index() was called with an empty corpus. Searching an empty index should return an empty result list, not crash.

The covering test is marked @pytest.mark.xfail referencing manifest id H-01 — remove the marker as part of the fix.

Relevant files:

• rag/retriever/keyword_search.py
• tests/unit/test_keyword_search.py

Estimated effort: 2–4 hours