refactor: add to template agent roles

sources/agents/google_base_chat.toml

@@ -3,12 +3,13 @@ schema_version = 1
 name        = "Google Base Chat"
 description = "Google Gemini generateContent request body. Abstract — extend it and set model/endpoint."
 abstract    = true
-extends     = "Chat Base"
 
 provider_instance = "Google AI"
 enable_tools      = true
 tags = ["chat", "gemini", "google", "cloud"]
 
+system_prompt = """{{ agent_role() }}"""
+
 [body]
 system_instruction = """{% if existsIn(ctx, "system_prompt") %}{ "parts": [ { "text": {{ tojson(ctx.system_prompt) }} } ] }{% endif %}"""
 contents = """