refactor: add to template agent roles

sources/agents/openai_base_chat.toml

@@ -3,16 +3,18 @@ schema_version = 1
 name        = "OpenAI Base Chat"
 description = "OpenAI Chat Completions request body. Abstract — extend it and set provider/endpoint/model."
 abstract    = true
-extends     = "Chat Base"
 
 provider_instance = "OpenAI (Chat Completions)"
 endpoint          = "/chat/completions"
 enable_tools      = true
 tags = ["chat", "openai", "cloud"]
 
+system_prompt = """{{ agent_role() }}"""
+
 [body]
 max_tokens  = 8192
 temperature = 0.7
+stream      = true
 messages    = """
 [ {% include "partials/openai_messages.jinja" %} ]
 """