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-bridge.ts

@@ -0,0 +1,117 @@
+import { Agent } from '@earendil-works/pi-agent-core';
+import type { AgentEvent } from '@earendil-works/pi-agent-core';
+import { streamSimple } from '@earendil-works/pi-ai';
+import type { AssistantMessage } from '@earendil-works/pi-ai';
+import { WebSocket } from 'ws';
+import { createPiModel } from './pi-model';
+import { createPiTools } from './pi-tools';
+import { tools } from './tools/index';
+import { videoAgent } from './index';
+import { dbToPiMessages, saveUserMessage, saveAssistantMessage, saveToolResult, type DbMessage } from './pi-persist';
+import { getDb } from '../db';
+
+interface RunContext {
+  currentAssistantMsgId: string | null;
+}
+
+export async function runAgentChat(ws: WebSocket, convId: string, userContent: string) {
+  const userMsgId = saveUserMessage(convId, userContent);
+  ws.send(JSON.stringify({ type: 'message', data: { id: userMsgId, role: 'user', content: userContent } }));
+
+  const msgCount = getDb().prepare('SELECT COUNT(*) as count FROM messages WHERE conversation_id = ?').get(convId) as { count: number };
+  if (msgCount.count <= 1) {
+    const title = userContent.slice(0, 30) + (userContent.length > 30 ? '...' : '');
+    getDb().prepare("UPDATE conversations SET title = ?, updated_at = datetime('now') WHERE id = ?").run(title, convId);
+  }
+  getDb().prepare("UPDATE conversations SET updated_at = datetime('now') WHERE id = ?").run(convId);
+
+  const history = getDb().prepare(
+    'SELECT * FROM messages WHERE conversation_id = ? AND id != ? ORDER BY created_at'
+  ).all(convId, userMsgId) as DbMessage[];
+  const piMessages = dbToPiMessages(history);
+
+  const { model, apiKey } = createPiModel();
+  const piTools = createPiTools(tools);
+
+  const agent = new Agent({
+    initialState: {
+      systemPrompt: videoAgent.getSystemPrompt(),
+      model,
+      thinkingLevel: 'off',
+      tools: piTools,
+      messages: piMessages,
+    },
+    streamFn: streamSimple,
+    getApiKey: () => apiKey,
+  });
+
+  const ctx: RunContext = { currentAssistantMsgId: null };
+
+  agent.subscribe((event: AgentEvent) => {
+    handleAgentEvent(ws, convId, event, ctx);
+  });
+
+  ws.send(JSON.stringify({ type: 'status', data: { status: 'thinking' } }));
+
+  try {
+    await agent.prompt(userContent);
+  } catch (err) {
+    const errMsg = (err as Error).message;
+    console.error('[pi-bridge] Agent error:', errMsg);
+    ws.send(JSON.stringify({
+      type: 'message',
+      data: { id: '', role: 'assistant', content: `抱歉，出错了：${errMsg}` },
+    }));
+  }
+}
+
+function handleAgentEvent(ws: WebSocket, convId: string, event: AgentEvent, ctx: RunContext) {
+  switch (event.type) {
+    case 'message_start': {
+      if (event.message.role === 'assistant') {
+        const id = crypto.randomUUID();
+        ctx.currentAssistantMsgId = id;
+        ws.send(JSON.stringify({ type: 'message_start', data: { id } }));
+      }
+      break;
+    }
+
+    case 'message_update': {
+      const piEvent = event.assistantMessageEvent;
+      const id = ctx.currentAssistantMsgId || '';
+
+      if (piEvent.type === 'text_delta') {
+        ws.send(JSON.stringify({ type: 'text_delta', data: { id, text: piEvent.delta } }));
+      } else if (piEvent.type === 'thinking_delta') {
+        ws.send(JSON.stringify({ type: 'reasoning_delta', data: { id, text: piEvent.delta } }));
+      }
+      break;
+    }
+
+    case 'message_end': {
+      if (event.message.role === 'assistant') {
+        const id = ctx.currentAssistantMsgId || '';
+        ws.send(JSON.stringify({ type: 'message_end', data: { id } }));
+        saveAssistantMessage(convId, event.message as AssistantMessage);
+        ctx.currentAssistantMsgId = null;
+      }
+      break;
+    }
+
+    case 'tool_execution_start': {
+      ws.send(JSON.stringify({ type: 'tool_start', data: { tool: event.toolName, input: event.args } }));
+      break;
+    }
+
+    case 'tool_execution_end': {
+      const resultText = event.result?.content?.map((c: any) => c.text || '').join('') || '';
+      if (event.isError) {
+        ws.send(JSON.stringify({ type: 'tool_error', data: { tool: event.toolName, error: resultText } }));
+      } else {
+        ws.send(JSON.stringify({ type: 'tool_result', data: { tool: event.toolName, result: resultText.slice(0, 1000) } }));
+      }
+      saveToolResult(convId, event.toolCallId, event.toolName, resultText, event.isError);
+      break;
+    }
+  }
+}