feat(#29): implement cron job configuration

- Add CronSchedule model to Prisma schema
- Implement CronService with CRUD operations
- Add REST API endpoints for cron management
- Create MoltBot plugin skill definition (SKILL.md)
- TDD: 9 passing tests for CronService

plugins/mosaic-plugin-cron/SKILL.md

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+---
+name: mosaic-plugin-cron
+description: Schedule recurring commands and reminders with cron expressions
+version: 0.0.1
+triggers:
+  - "schedule a reminder"
+  - "create cron job"
+  - "list my schedules"
+  - "delete schedule"
+  - "enable schedule"
+  - "disable schedule"
+tools:
+  - mosaic_cron_api
+---
+
+# Mosaic Cron Plugin
+
+Schedule recurring commands and reminders for your workspace using cron expressions.
+
+## Usage Examples
+
+- "Schedule a reminder at 9am every day: morning briefing"
+- "Create a cron job: 0 17 * * 1-5 for daily standup"
+- "Show all my schedules"
+- "Delete the 9am daily reminder"
+
+## Cron Expression Format
+
+Standard cron format: `minute hour day-of-month month day-of-week`
+
+| Field | Values | Example |
+|-------|--------|---------|
+| Minute | 0-59 | `0` = top of hour |
+| Hour | 0-23 | `9` = 9am |
+| Day of Month | 1-31 | `*` = every day |
+| Month | 1-12 | `*` = every month |
+| Day of Week | 0-6 | `1-5` = Mon-Fri |
+
+### Common Examples
+
+- `0 9 * * *` - Every day at 9am
+- `0 8 * * 1` - Every Monday at 8am
+- `0 17 * * 1-5` - Mon-Fri at 5pm
+- `0 0 1 * *` - First day of every month at midnight
+- `*/15 * * * *` - Every 15 minutes
+
+## API Endpoints
+
+All cron operations are available via the Mosaic API:
+
+- `POST /api/cron` - Create schedule
+- `GET /api/cron` - List schedules
+- `GET /api/cron/:id` - Get schedule
+- `PATCH /api/cron/:id` - Update schedule
+- `DELETE /api/cron/:id` - Delete schedule
+
+## Permissions
+
+- **Create/Update schedules**: Workspace MEMBER or higher
+- **Delete schedules**: Workspace ADMIN or higher
+- **View schedules**: Any workspace member