refactor(messaging): 拆分用户交互模块到 application/messaging 层

- 新增 application/messaging 交互层：router.py 统一会话优先级与回调分发，
  site/subscribe/skill/media/plugin 各交互状态与视图从 Chain 迁出
- MessageChain 改为通过 InteractionRouter 派发文本会话与按钮回调，
  新增结构化 callback_data 通道（兼容 CALLBACK: 文本前缀）
- Transfer 失败重试/AI 接管回调归入 TransferChain
- MediaInteractionChain 拆出为 app/chain/interaction.py（旧路径保留兼容别名）
- WebAgent Endpoint 去重，统一使用 agent.py 回调协议函数
- 删除 app/chain/skills.py（交互逻辑并入 SkillInteractionHandler）
- 同步更新架构文档与测试，全量 4476 通过

docs/rules/01-project-overview.md

@@ -32,7 +32,7 @@ MoviePilot is a self-hosted media automation platform targeting Chinese-language
 | `app/runtime/` | Config, events, logging, caching, concurrency, process state, extensions, and legacy compatibility |
 | `app/adapters/` | Cache, network, system, generated-resource, and named external-product adapters |
 | `app/runtime/extensions/` | Module, plugin, and configured-service lifecycle management |
-| `app/application/messaging/` | Messaging, interaction, and Agent-to-message capabilities |
+| `app/application/messaging/` | Messaging, interaction, and Agent-to-message capabilities (`interaction.py` contracts, `router.py` priority/callback dispatch, `site.py`/`subscribe.py`/`skill.py` command sessions, `media.py` media interaction state, `plugin.py` plugin input, `agent.py` agent choice bridge, `message.py` rendering and queue); not a public plugin SDK |
 | `app/application/security/` | Authentication and access-control capabilities |
 | `app/application/` | Focused application services |
 | `app/sdk/` | Stable imports for plugins |