suppressing prompt field in agent config doc

Since the feature and the main docs are live, let's not include this yet, at least not right here on the page with the live features.

Author: mibeco

docs/agent-format.md

@@ -38,15 +38,7 @@ The `description` field provides a description of what the agent does. This is p
 }
 ```
 
-## Prompt Field
 
-The `prompt` field is intended to provide high-level context to the agent, similar to a system prompt. This feature is not yet implemented.
-
-```json
-{
-  "prompt": "You are an expert AWS infrastructure specialist"
-}
-```
 
 ## McpServers Field