fix: Handle hook return values to enable context modification

The plugin hook system calls _hook('agent_before', locals()) but was
not handling the return value. This prevented plugins from modifying
the agent context (e.g., system_prompt).

This fix:
- Captures the hook return value
- Applies modifications when a dict is returned
- Maintains backward compatibility with hooks that don't return values

Fixes #537

Author: Hanson Mei

agent_loop.py

@@ -46,7 +46,11 @@ def agent_runner_loop(client, system_prompt, user_input, handler, tools_schema,
         {"role": "user", "content": initial_user_content if initial_user_content is not None else user_input}
     ]
     turn = 0;  handler.max_turns = max_turns
-    _hook('agent_before', locals())
+    result = _hook('agent_before', locals())
+    if isinstance(result, dict):
+        # Hook 可能修改了 locals，需要更新变量
+        if 'system_prompt' in result:
+            messages[0]['content'] = result['system_prompt']
     while turn < handler.max_turns:
         turn += 1; turnstr = f'LLM Running (Turn {turn}) ...'
         if handler.parent.task_dir: turnstr = f'Turn {turn} ...'