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/plugins/common.ts

@@ -0,0 +1,31 @@
+import path from 'node:path';
+
+export const MAX_SECRET_BYTES = 64 * 1024;
+export const MAX_CONFIG_BYTES = 512 * 1024;
+export const MAX_SOURCE_BYTES = 2 * 1024 * 1024;
+
+export function asError(error: unknown): Error {
+  return error instanceof Error ? error : new Error(String(error));
+}
+
+export function errorMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error);
+}
+
+export function hasCode(error: unknown, code: string): boolean {
+  return typeof error === 'object' && error !== null && 'code' in error && error.code === code;
+}
+
+export function samePath(left: string, right: string): boolean {
+  const normalizedLeft = path.resolve(left);
+  const normalizedRight = path.resolve(right);
+  return process.platform === 'win32'
+    ? normalizedLeft.toLowerCase() === normalizedRight.toLowerCase()
+    : normalizedLeft === normalizedRight;
+}
+
+export function sameAgentId(left: unknown, right: string): boolean {
+  return typeof left === 'string' && (process.platform === 'win32'
+    ? left.toLowerCase() === right.toLowerCase()
+    : left === right);
+}