refactor(settings): deprecate SAGEMAKER_* and drop two unused settings

SAGEMAKER_REGION, SAGEMAKER_ACCESS_KEY and SAGEMAKER_SECRET_KEY survive
only as a fallback for the S3_* credentials. They carry
Field(deprecated=...) now, so any read emits a DeprecationWarning naming
the replacement and the generated reference shows the notice. The S3
store is the one sanctioned reader; it silences that warning locally
because it already logs its own operator-facing one when the fallback
is actually used.

DEFAULT_MAX_HISTORY was referenced nowhere. RETRIEVERS_ENABLED was read by
no code at all, while two docs pages described it as an enforced
allow-list; both the setting and those claims are removed.

docs/content/Sources/Per-source-configuration.mdx

@@ -115,8 +115,6 @@ Requests are also bounded: `chunks` is clamped to 0–500, and `0` still means
 Keyword search for the **hybrid** retriever is currently implemented only for the **pgvector** vector store. On other stores (FAISS, Qdrant, Milvus, etc.) the keyword half returns nothing, so `hybrid` quietly behaves like `classic` (vector-only).
 </Callout>
 
-Operators can restrict which retrievers are usable instance-wide with the `RETRIEVERS_ENABLED` setting; a per-source `retriever` value must be within that allow-list.
-
 ### Exposure: prefetch vs. agentic tool
 
 `exposure` controls *how* a source's content is delivered to the model: