Cohere reranking

@mayurajs/reranker-cohere implements the RAG contracts. Install it alongside mayura. Construction performs no I/O, credential discovery, provisioning or background work. All operations require an explicit host permission and a nonnegative costMicros ceiling. Offline tests and package checks passed locally. Real service behavior remains unqualified; see release validation status.

cohereReranker accepts apiKey, model, costMicros, permissions, timeoutMs, maxConcurrent and optional test fetch. Grant rag:transport:cohere on the adapter and rag:rerank:cohere on the retriever. The endpoint is pinned to the native service; no automatic retries or redirects run. The retriever also requires an explicit hosted sensitivity profile before disclosing authorized document text.

One request ranks at most 100 original documents. The adapter validates unique indexes and finite scores and returns only index/score pairs. Provider-returned document text cannot amend or inject evidence. The declared maximum is reserved even when a service fails; configure the ceiling for the admitted batch and model.

Protocol reference, reviewed 2026-10-20: cohere documentation.