feat(rag): add pluggable reranker interface [BACKLOG]

[however-yir]

Problem

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

Status: BACKLOG

Not implementing in current cycle. Note: a basic IdentityReranker already exists that serves as the no-op default.

Deferred until

• At least two reranker backends are identified for integration
• Performance benchmarks for reranking latency vs. quality are available
• Retrieval pipeline refactoring is complete

Original scope

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

[however-yir]

Update: Interface implemented, tests added

Acceptance criteria status:

• Reranker abstraction is documented → Reranker.java interface with rerank(query, documents, topK) and getName() methods
• Default no-op reranker keeps existing behavior → IdentityReranker.java (@Component, passes through documents with topK limit)
• Tests cover ranking order and fallback behavior → IdentityRerankerTest.java with 5 test cases (topK truncation, empty input, zero topK, oversized topK, name verification)

Implementation complete.