refactor(chain): 处理链功能域 mixin 化，清理未使用导入并根治兼容层循环导入

- ChainBase 拆分为 RecognitionMixin/MessageProcessingMixin/NotificationMixin
- TransferChain 拆分为 7 个功能 mixin（_mixins.py），SubscribeChain 音乐订阅域拆出 _music.py
- 斜杠命令交互四件套收敛为 InteractionChainMixin 委托，会话管理器移至 application 层，chain 层不再 re-export
- 模块基础类收敛到 app/modules/_base（notification/mediaserver 语义重命名）
- 清理 app/chain/__init__.py 24 个未使用导入，修正 49 处测试 patch 目标到实际命名空间
- 兼容层 legacy 符号不再并入 __all__，根治 schemas 初始化反向拉起 application.transfer 的循环导入
- 修复 bangumi 集数为字符串时 set_bangumi_info 抛 TypeError
- 新增重复代码等架构门禁测试；capability 清单校验排除下划线内部目录

app/agent/orchestrator.py

@@ -6,7 +6,6 @@ import traceback
 import uuid
 from dataclasses import dataclass
 from datetime import datetime, timedelta
-from enum import Enum
 from typing import Any, Callable, Dict, List, Optional
 
 from fastapi.concurrency import run_in_threadpool
@@ -68,7 +67,7 @@ from app.agent.tools.impl.mcp import (
     select_legacy_mcp_tools,
 )
 from app.agent.tools.impl.query_system_settings import QuerySystemSettingsTool
-from app.chain import ChainBase
+from app.chain.agent import AgentChain
 from app.runtime.config import settings
 from app.runtime.events import eventmanager
 from app.runtime.extensions.plugin_manager import PluginManager
@@ -77,17 +76,12 @@ from app.db.oper.agenttask import AgentTaskOper
 from app.db.oper.user import UserOper
 from app.runtime.log import logger
 from app.schemas import AgentLLMProviderEventData, AgentTokensUsageEventData, Notification, NotificationType
+from app.schemas.agent import ReplyMode
 from app.schemas.message import ChannelCapabilityManager, ChannelCapability
 from app.schemas.types import ChainEventType, EventType, MessageChannel
 from app.foundation.identity import SYSTEM_INTERNAL_USER_ID
 
 
-class AgentChain(ChainBase):
-    """Agent 业务处理链。"""
-
-    pass
-
-
 def _finish_processing_status(status: Optional[dict], user_id: Optional[str] = None) -> None:
     """结束入站消息的渠道处理状态。"""
     if not status:
@@ -321,15 +315,6 @@ class _ThinkTagStripper:
             self.buffer = ""
 
 
-class ReplyMode(str, Enum):
-    """
-    Agent 最终回复处理模式。
-    """
-
-    DISPATCH = "dispatch"
-    CAPTURE_ONLY = "capture_only"
-
-
 HEARTBEAT_SESSION_PREFIX = "__agent_heartbeat_"
 UNSUPPORTED_IMAGE_INPUT_MESSAGE = "当前模型不支持图片输入，请更换支持图片输入的模型，或在系统设置中关闭图片输入支持后重试。"
 AGENT_EXECUTION_ERROR_PREFIX = "智能助手执行失败"