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/server/agent/pi-tools.ts

@@ -0,0 +1,18 @@
+import type { AgentTool, AgentToolResult } from '@earendil-works/pi-agent-core';
+import type { ToolDefinition } from './tools/types';
+
+export function createPiTools(tools: ToolDefinition[]): AgentTool[] {
+  return tools.map((t): AgentTool => ({
+    name: t.name,
+    description: t.description,
+    parameters: t.input_schema as any,
+    label: t.name,
+    execute: async (_toolCallId: string, params: any): Promise<AgentToolResult<any>> => {
+      const result = await t.execute(params);
+      return {
+        content: [{ type: 'text' as const, text: result }],
+        details: null,
+      };
+    },
+  }));
+}