feat(Core): expose CronExpression via motia and update cron example

[Yusuf-96]

Summary

Introduced CronExpression export from packages/core/index.ts so consumers importing from motia can use the enum directly instead of deep paths into core internals.
Updated playground/steps/cronExample/handlePeriodicJob.step.ts to consume CronExpression from motia, showing that either the enum or a raw cron string works in cron configs and keeping the example aligned with the new public API.
Simplify cron usage for app developers by exposing the enum at the package root and demonstrating the preferred import pattern in the playground cron example.

Related Issues

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactor
• Other (please describe):

Checklist

• I have read the CONTRIBUTING.md
• My code follows the code style of this project
• I have added tests where applicable
• I have tested my changes locally
• I have linked relevant issues
• I have added screenshots for UI changes (if applicable)

Screenshots (if applicable)

Additional Context

No automated tests were run for these changes, as they only adjust exports and a playground example.

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