add TREC RAG 2026 answer capability

[nourj98]

Summary

Adds end-to-end support for the official TREC RAG 2026 answer task over ClimbMix-400b.

Changes

• Registers the official 119-topic trec-rag-2026 benchmark as an unjudged collection.
• Makes qrels optional and skips retrieval evaluation when judgments are unavailable.
• Adds the TREC RAG answer prompt and validation:
  • Sentence-level answers with ClimbMix document-ID citations
  • Maximum 1,024 words
  • Maximum three citations per sentence
  • Citation/reference consistency checks
  • Deterministically generated run metadata
• Adds team_id, run_id, and run_desc configuration.
• Persists validated structured output in per-query artifacts.
• Generates the official rag_output_trec_rag_2026.jsonl submission.
• Improves sharded merging, completion checks, resumability, and failed-topic retries.
• Adds retry/backoff handling for transient Pyserini REST errors.
• Updates documentation and focused tests.

Validation

• Verified authenticated retrieval against climbmix-400b.
• Completed an isolated rag2026-0 smoke test.
• Validated metadata, narratives, word limits, citation structure, and referenced document IDs.
• Ran focused tests, type checking, linting, and formatting checks.

[lilyjge]

roughly lgtm but I'm not the most familiar with typescript, @ricky42613 ptal?