refactor(schemas): 统一 message/notification 命名边界，旧名收敛至兼容映射表

- notification 域：渠道能力（MessageChannel→NotificationChannel、ChannelCapability* 迁入 notification.py）
- message 域：消息收发（Notification→Message、NotificationType→MessageType、CommingMessage→IncomingMessage、NotificationHistoryItem→MessageHistoryItem、NotificationClear*→MessageClear*）
- Agent 工具契约：send_notification_message→send_message、notification_callback→message_callback
- 源码不保留旧名物理别名，旧导入经 app/runtime/compat/manifest.py SYMBOL_ALIASES 惰性解析
- API 路径与持久化键冻结不变，前端零改动
- 新增兼容守护测试与 docs/rules/07 命名边界规范

tests/test_system_notification_dispatch.py

@@ -12,7 +12,7 @@ sys.modules.setdefault("psutil", ModuleType("psutil"))
 
 from app.chain.message import MessageChain
 from app.application.messaging.message import MessageQueueManager
-from app.schemas import Notification
+from app.schemas import Message
 from app.foundation.identity import (
     SYSTEM_INTERNAL_USER_ID,
     is_internal_user_id,
@@ -29,7 +29,7 @@ class TestSystemNotificationDispatch(unittest.TestCase):
 
     def test_post_message_normalizes_internal_userid_before_queueing(self):
         chain = MessageChain()
-        message = Notification(
+        message = Message(
             userid=SYSTEM_INTERNAL_USER_ID,
             username="admin",
             title="后台报告",
@@ -54,7 +54,7 @@ class TestSystemNotificationDispatch(unittest.TestCase):
 
     def test_send_direct_message_normalizes_internal_userid(self):
         chain = MessageChain()
-        message = Notification(
+        message = Message(
             userid=SYSTEM_INTERNAL_USER_ID,
             username="admin",
             title="后台报告",