feat(agent): Add configurable system_prompt_filename to AgentConfig (#9265)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/agenthub/codeact_agent/codeact_agent.py

@@ -95,6 +95,7 @@ class CodeActAgent(Agent):
         if self._prompt_manager is None:
             self._prompt_manager = PromptManager(
                 prompt_dir=os.path.join(os.path.dirname(__file__), 'prompts'),
+                system_prompt_filename=self.config.system_prompt_filename,
             )
 
         return self._prompt_manager