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>

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+  },
+})