refactor(agent): 将 tools 模块拆分为独立文件并优化导入路径

将 `tools.ts` 拆分为按功能划分的独立文件，并存放于 `tools/` 目录下，同时更新导入路径；优化 agent 系统提示语，移除冗余的「美图 Agent」前缀。

web/server/agent/tools/get-manifest.ts

@@ -0,0 +1,25 @@
+import path from 'path';
+import fs from 'fs';
+import { PROJECT_ROOT, loadJSON } from './shared';
+import type { ToolDefinition } from './types';
+
+export const getManifest: ToolDefinition = {
+  name: 'get_manifest',
+  description: '读取指定 manifest.json 的完整内容，返回 JSON 字符串。',
+  input_schema: {
+    type: 'object',
+    properties: {
+      manifestPath: { type: 'string', description: 'manifest.json 的绝对路径或相对路径' },
+    },
+    required: ['manifestPath'],
+  },
+  execute: async (params) => {
+    const { manifestPath } = params as { manifestPath: string };
+    const resolved = path.isAbsolute(manifestPath)
+      ? manifestPath
+      : path.resolve(PROJECT_ROOT, manifestPath);
+    if (!fs.existsSync(resolved)) return `manifest 不存在: ${resolved}`;
+    const manifest = loadJSON(resolved);
+    return JSON.stringify(manifest, null, 2);
+  },
+};