Add evidence-aware retrieval responses

[MohammadSyed18]

What changed

• map every generated sentence to its supporting source citation
• return structured evidence with passage IDs and retrieval scores
• detect conflicting dates across retrieved evidence and reduce confidence

Why

Answers previously numbered sentences independently from deduplicated citations, which could make a marker point at the wrong source. Explicit evidence mapping makes retrieval output auditable and conflicting announcements visible.

Validation

• npm test (6 tests)
• git diff --check