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/Deploying/DocsGPT-Settings.mdx

@@ -461,7 +461,6 @@ These control how sources are retrieved and whether the advanced RAG features ar
 
 | Setting | Default | Description |
 | --- | --- | --- |
-| `RETRIEVERS_ENABLED` | `["classic", "default"]` | Allow-list of retrievers usable instance-wide. Valid keys: `classic`, `default`, `hybrid`, `graphrag`. A per-source `retriever` must be within this list. |
 | `PER_SOURCE_RETRIEVAL_ENABLED` | `true` | Master switch for per-source retrieval config. When `false`, all sources fall back to the classic retriever regardless of their stored config. |
 | `GRAPHRAG_ENABLED` | `false` | Enable [GraphRAG](/Sources/GraphRAG). Requires `VECTOR_STORE=pgvector`. |
 | `GRAPHRAG_EXTRACTION_MODEL` | unset | Model used for ingest-time graph extraction. Unset reuses the instance default model. |

docs/content/Deploying/Settings-Reference.mdx

@@ -417,12 +417,6 @@ Type `"faiss" | "elasticsearch" | "mongodb" | "qdrant" | "milvus" | "pgvector"`,
 
 Vector store backend.
 
-### `RETRIEVERS_ENABLED`
-
-Type `list`, default `["classic", "default"]`.
-
-Retriever keys an agent may use; must match RetrieverCreator.retrievers registry keys, NOT the legacy classic_rag label which never matched the registry.
-
 ### `RETRIEVAL_MAX_PARALLEL_SOURCES`
 
 Type `int`, default `4`, must be >= 1.
@@ -1015,18 +1009,24 @@ Path-style addressing (required by most non-AWS services).
 
 Type `str`, default unset.
 
+**Deprecated.** Set S3_REGION instead; the SAGEMAKER_* fallback will be removed.
+
 Legacy AWS region from the retired SageMaker provider; deprecated fallback for S3_REGION.
 
 ### `SAGEMAKER_ACCESS_KEY`
 
 Type `str`, default unset.
 
+**Deprecated.** Set S3_ACCESS_KEY_ID instead; the SAGEMAKER_* fallback will be removed.
+
 Legacy AWS access key from the retired SageMaker provider; deprecated fallback for S3_ACCESS_KEY_ID.
 
 ### `SAGEMAKER_SECRET_KEY`
 
 Type `str`, default unset.
 
+**Deprecated.** Set S3_SECRET_ACCESS_KEY instead; the SAGEMAKER_* fallback will be removed.
+
 Legacy AWS secret key from the retired SageMaker provider; deprecated fallback for S3_SECRET_ACCESS_KEY.
 
 
@@ -1271,12 +1271,6 @@ Type `str`, default `classic`.
 
 Default agent type for agentless chats.
 
-### `DEFAULT_MAX_HISTORY`
-
-Type `int`, default `150`.
-
-Default number of history messages kept.
-
 ### `DEFAULT_AGENT_LIMITS`
 
 Type `dict`, default `{"token_limit": 50000, "request_limit": 500}`.

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: