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

test/guard-template.test.mjs

@@ -6,7 +6,7 @@ import os from 'node:os';
 import path from 'node:path';
 import test from 'node:test';
 
-import { generateGuardScript } from '../dist/plugins/hook-template.js';
+import { generateGuardScript } from '../dist/plugins/guard-template.js';
 
 const AGENT_ID = 'agent-1';