feat: 行程规划改用 tool-calling agent，旧逻辑保留为降级方案

ai.ts 新增通用 runAgentLoop()，blindboxPlanGen.ts 拆分为
agent 主路径（list_ideas/search_poi/finalize_plan 三个工具）
和 legacy 降级路径，agent 失败时自动回退。
AI 思考和工具调用实时推送给前端。

src/lib/ai.ts

@@ -1,4 +1,8 @@
 import OpenAI from "openai";
+import type {
+  ChatCompletionMessageParam,
+  ChatCompletionTool,
+} from "openai/resources/chat/completions";
 import type { IdeaTags, PlanItem } from "@/types";
 
 function getClient() {
@@ -224,3 +228,133 @@ ${Object.entries(ctx.candidates)
     return null;
   }
 }
+
+// ---------------------------------------------------------------------------
+// Generic tool-calling agent loop
+// ---------------------------------------------------------------------------
+
+export interface AgentTool {
+  name: string;
+  description: string;
+  parameters: Record<string, unknown>; // JSON Schema for the tool parameters
+  execute: (args: Record<string, unknown>) => Promise<string>;
+  progressBefore?: (args: Record<string, unknown>) => string;
+  progressAfter?: (args: Record<string, unknown>, result: string) => string;
+}
+
+export interface AgentLoopOptions {
+  systemPrompt: string;
+  userPrompt: string;
+  tools: AgentTool[];
+  onProgress?: (message: string) => void;
+  maxTurns?: number;
+}
+
+export interface AgentLoopResult {
+  finalArgs: Record<string, unknown>;
+  turns: number;
+}
+
+export async function runAgentLoop(
+  options: AgentLoopOptions,
+): Promise<AgentLoopResult | null> {
+  const { systemPrompt, userPrompt, tools, onProgress, maxTurns = 15 } = options;
+  const client = getClient();
+
+  const openaiTools: ChatCompletionTool[] = tools.map((t) => ({
+    type: "function" as const,
+    function: {
+      name: t.name,
+      description: t.description,
+      parameters: t.parameters,
+    },
+  }));
+
+  const toolMap = new Map(tools.map((t) => [t.name, t]));
+
+  const messages: ChatCompletionMessageParam[] = [
+    { role: "system", content: systemPrompt },
+    { role: "user", content: userPrompt },
+  ];
+
+  for (let turn = 0; turn < maxTurns; turn++) {
+    const response = await client.chat.completions.create({
+      model: "deepseek-chat",
+      messages,
+      tools: openaiTools,
+      temperature: 0.6,
+      max_tokens: 4096,
+    });
+
+    const choice = response.choices[0];
+    if (!choice) break;
+
+    const assistantMessage = choice.message;
+
+    // Push AI thinking text to the user
+    if (assistantMessage.content) {
+      onProgress?.(assistantMessage.content);
+    }
+
+    // No tool calls — conversation ended
+    if (!assistantMessage.tool_calls || assistantMessage.tool_calls.length === 0) {
+      // Add the assistant message and break if there's nothing more to do
+      messages.push({ role: "assistant", content: assistantMessage.content ?? "" });
+      if (choice.finish_reason === "stop") break;
+      continue;
+    }
+
+    // Add the assistant message (with tool_calls) to history
+    messages.push(assistantMessage as ChatCompletionMessageParam);
+
+    // Execute each tool call
+    for (const toolCall of assistantMessage.tool_calls) {
+      if (toolCall.type !== "function") continue;
+      const fnName = toolCall.function.name;
+      const tool = toolMap.get(fnName);
+      if (!tool) {
+        messages.push({
+          role: "tool",
+          tool_call_id: toolCall.id,
+          content: JSON.stringify({ error: `Unknown tool: ${fnName}` }),
+        });
+        continue;
+      }
+
+      let args: Record<string, unknown>;
+      try {
+        args = JSON.parse(toolCall.function.arguments);
+      } catch {
+        args = {};
+      }
+
+      // Progress: before execution
+      if (tool.progressBefore) {
+        onProgress?.(tool.progressBefore(args));
+      }
+
+      // Execute
+      const result = await tool.execute(args);
+
+      // Progress: after execution
+      if (tool.progressAfter) {
+        onProgress?.(tool.progressAfter(args, result));
+      }
+
+      // If this is finalize_plan, return immediately
+      if (fnName === "finalize_plan") {
+        return { finalArgs: args, turns: turn + 1 };
+      }
+
+      // Feed tool result back to the conversation
+      messages.push({
+        role: "tool",
+        tool_call_id: toolCall.id,
+        content: result,
+      });
+    }
+  }
+
+  // Exhausted turns without finalize
+  return null;
+}