feat(rag): add pluggable reranker interface

[however-yir]

Problem

The RAG pipeline should allow teams to compare rerankers without rewriting retrieval and answer-generation code.

Scope

• Define a reranker interface around retrieved chunks.
• Keep the default behavior backward compatible.
• Add configuration hooks for enabling/disabling reranking.

Acceptance criteria

• Reranker abstraction is documented.
• Default no-op reranker keeps existing behavior.
• Tests cover ranking order and fallback behavior.