Implement query expansion with multi-query hybrid retrieval and RRF fusion

[aurexav]

Context
We want higher recall and fewer misses than single-query hybrid retrieval. QMD expands the query and fuses multiple ranked lists with RRF.

Goal
Increase recall without sacrificing precision by expanding queries and fusing results deterministically.

Scope

• Add a query expansion step with configurable provider and expansion count.
• Run hybrid retrieval for the original query and expansions.
• Fuse results with RRF and weight the original query higher.
• Keep the existing rerank step and tie-breaker logic.

Acceptance Criteria

• Recall improves on a representative evaluation set.
• Disabling expansion preserves current behavior.

References

• https://github.com/tobi/qmd

[aurexav]

Closed by commit 0d94607 on main.