feat: Add AGENTS.md auto-injection via plugin system

- Add plugins/agents_loader.py: Auto-inject AGENTS.md into system prompt via agent_before hook
- Fix agent_loop.py: Handle hook return value to update system_prompt
- Bug fixes found during implementation:
  * hooks.py uses 'register' decorator, not 'on'
  * Hook must return modified context dict to take effect

This enables project-specific constraints in AGENTS.md to be automatically
loaded without modifying core source code.

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} ...'

plugins/agents_loader.py

@@ -0,0 +1,23 @@
+"""
+AGENTS.md loader plugin - 在 agent_before 时注入 AGENTS.md 到 system prompt
+"""
+import os
+from plugins import hooks
+
+_PROJECT_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+_AGENTS_PATH = os.path.join(_PROJECT_ROOT, 'AGENTS.md')
+
+# 加载 AGENTS.md 内容
+_AGENTS_CONTENT = None
+if os.path.exists(_AGENTS_PATH):
+    with open(_AGENTS_PATH, 'r', encoding='utf-8') as f:
+        _AGENTS_CONTENT = f.read()
+
+@hooks.register('agent_before')
+def inject_agents_md(ctx):
+    """在 agent 启动前注入 AGENTS.md 到 system prompt"""
+    if _AGENTS_CONTENT and 'system_prompt' in ctx:
+        # 在 system prompt 后面追加 AGENTS.md 内容
+        ctx['system_prompt'] += f'\n\n[Project Constraints] (../AGENTS.md)\n{_AGENTS_CONTENT}\n'
+        return ctx
+    return ctx