chore(node-sdk): audit plugins shared

Add plugins/common.ts for the helpers and size limits copied across plugin files; managed-transaction compares snapshots in one place; guard template imported directly.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

src/cli.ts

@@ -13,7 +13,8 @@ import {
 import { writePrivateFile } from './secure-files.js';
 import { SUPPORTED_AGENTS } from './plugins/registry.js';
 import type { AgentPlugin, InstallConfig } from './plugins/base.js';
-import { generateHookScript, generateGuardScript } from './plugins/hook-template.js';
+import { generateGuardScript } from './plugins/guard-template.js';
+import { generateHookScript } from './plugins/hook-template.js';
 import { augmentPlugin } from './plugins/augment.js';
 import { claudecodePlugin } from './plugins/claudecode.js';
 import { cursorPlugin } from './plugins/cursor.js';