Fix client-side bundling by separating server and client exports

- Create dedicated /server export for server-side functions and plugin
- Main export now contains only types (client-safe)
- Remove problematic /steps export that exposed server functions
- Update README with correct import structure
- Fix tests to use /server export

This prevents server-side code from being bundled in client JavaScript,
eliminating the "require is not defined in ES module scope" runtime error.

Breaking change: workflowsPlugin must now be imported from '/server':
import { workflowsPlugin } from '@xtr-dev/payload-automation/server'

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/exports/server.ts

@@ -0,0 +1,26 @@
+// Server-side only exports - should never be bundled for client
+// These contain Node.js dependencies and should only be used server-side
+
+export { triggerCustomWorkflow, triggerWorkflowById } from '../core/trigger-custom-workflow.js'
+export { WorkflowExecutor } from '../core/workflow-executor.js'
+export { workflowsPlugin } from '../plugin/index.js'
+
+// Export all step handlers (server-side only)
+export {
+  createDocumentHandler,
+  deleteDocumentHandler,
+  httpStepHandler,
+  readDocumentHandler,
+  sendEmailHandler,
+  updateDocumentHandler
+} from '../steps/index.js'
+
+// Export step tasks configurations (server-side only)
+export {
+  CreateDocumentStepTask,
+  DeleteDocumentStepTask,
+  HttpRequestStepTask,
+  ReadDocumentStepTask,
+  SendEmailStepTask,
+  UpdateDocumentStepTask
+} from '../steps/index.js'