refactor(agent): 迁移AI会话引擎至pi-agent-core库

将原有基于Anthropic/OpenAI SDK的直播聊天代理重构为使用`@earendil-works/pi-agent-core`和`@earendil-works/pi-ai`库的统一API。

新增pi-bridge、pi-model、pi-persist、pi-tools四个模块，封装Agent路由、模型配置、消息持久化和工具适配逻辑。移除`chat.ts`中大量死代码，简化WebSocket处理流程。

BREAKING CHANGE: 移除`VideoAgent`类的`getAnthropicClient`、`getOpenAIClient`、`executeTool`等方法，外部调用需迁移至新pi-bridge API。`PROJECT_ROOT`路径计算方式变更，从`../../..`变为`../../`。

web/package.json

@@ -12,6 +12,8 @@
   },
   "dependencies": {
     "@anthropic-ai/sdk": "^0.95.0",
+    "@earendil-works/pi-agent-core": "^0.74.0",
+    "@earendil-works/pi-ai": "^0.74.0",
     "@types/multer": "^2.1.0",
     "better-sqlite3": "^11.6.0",
     "cors": "^2.8.5",