docs(runtime): clarify temp workspace ownership

Document the single-OS-user product boundary beside the default private temp root so future reviews do not treat unsupported multi-UID sharing as a regression.

packages/node-runtime/src/temp-workspace.ts

@@ -22,6 +22,8 @@ export function resolveChatLabTempRoot(options: ChatLabTempRootOptions = {}): st
   const envRoot = optionsEnv(options)[CHATLAB_TEMP_ROOT_ENV]?.trim()
   if (envRoot) return path.resolve(envRoot)
 
+  // Product boundary: ChatLab runs as a single OS user. The single private 0700 root is intentional;
+  // multi-UID hosts are unsupported, while isolated tests and CI must override CHATLAB_TEMP_ROOT.
   const platform = options.platform ?? process.platform
   const systemTempDir = options.systemTempDir ?? os.tmpdir()
   return path.resolve(platform === 'darwin' ? '/private/tmp/chatlab' : path.join(systemTempDir, 'chatlab'))