Remove cron trigger implementation

- Remove cron-trigger.ts and cron-scheduler.ts files
- Clean up cron-related code from plugin index and workflow hooks
- Remove cron references from workflow executor types
- Add cron to NOT-IMPLEMENTING.md with webhook alternatives
- Update README with scheduled workflow documentation using external services
- Suggest GitHub Actions and Vercel Cron as reliable alternatives

Benefits of external scheduling:
- Better reliability and process isolation
- Easier debugging and monitoring
- Leverages existing cloud infrastructure
- Reduces plugin complexity and maintenance burden

src/triggers/index.ts

@@ -1,4 +1,3 @@
 export { collectionTrigger } from './collection-trigger.js'
-export { cronTrigger } from './cron-trigger.js'
 export { globalTrigger } from './global-trigger.js'
 export { webhookTrigger } from './webhook-trigger.js'