docs: Remove unsupported payloadSizeThreshold from JSON stdin config examples

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

README.md

@@ -128,8 +128,7 @@ For the complete JSON configuration specification with all validation rules, see
     "domain": "localhost",
     "startupTimeout": 30,
     "toolTimeout": 60,
-    "payloadDir": "/tmp/jq-payloads",
-    "payloadSizeThreshold": 1024
+    "payloadDir": "/tmp/jq-payloads"
   }
 }
 ```
@@ -198,13 +197,16 @@ See **[Configuration Specification](https://github.com/github/gh-aw/blob/main/do
   - Must be positive integer
 - **`payloadDir`** (optional): Directory for storing large payload files (default: `/tmp/jq-payloads`)
   - Payloads are organized by session: `{payloadDir}/{sessionID}/{queryID}/payload.json`
-- **`payloadSizeThreshold`** (optional): Size threshold in bytes for payload storage (default: 1024)
-  - Payloads **larger** than this threshold are stored to disk and return metadata
-  - Payloads **smaller than or equal** to this threshold are returned inline
-  - Common values: `512` (aggressive), `1024` (default), `2048` (balanced), `10240` (minimal storage)
 
 **Note**: Gateway configuration fields are validated and parsed but not yet fully implemented.
 
+**Configuration Alternatives**:
+- **`payloadSizeThreshold`** is not supported in JSON stdin format. Use:
+  - CLI flag: `--payload-size-threshold <bytes>` (default: 1024)
+  - Environment variable: `MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD=<bytes>`
+  - Payloads **larger** than this threshold are stored to disk and return metadata
+  - Payloads **smaller than or equal** to this threshold are returned inline
+
 **Environment Variable Features**:
 - **Passthrough**: Set value to empty string (`""`) to pass through from host
 - **Expansion**: Use `${VAR_NAME}` syntax for dynamic substitution (fails if undefined)