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/index.ts

@@ -0,0 +1,28 @@
+import type { ToolDefinition } from './types';
+export type { ToolDefinition } from './types';
+
+import { listAccounts } from './list-accounts';
+import { createAccount } from './create-account';
+import { getAccountConfig } from './get-account-config';
+import { getAccountPrompts } from './get-account-prompts';
+import { pipelineStatus } from './pipeline-status';
+import { runPipelinePhase } from './run-pipeline-phase';
+import { generateImages } from './generate-images';
+import { generateVideos } from './generate-videos';
+import { listOutputs } from './list-outputs';
+import { getManifest } from './get-manifest';
+import { createManifest } from './create-manifest';
+
+export const tools: ToolDefinition[] = [
+  listAccounts,
+  createAccount,
+  getAccountConfig,
+  getAccountPrompts,
+  pipelineStatus,
+  runPipelinePhase,
+  generateImages,
+  generateVideos,
+  listOutputs,
+  getManifest,
+  createManifest,
+];