feat: add reminder system with cron scheduler and fix timezone formatting

- Add reminders to Prisma schema (Reminder model, TaskReminder relation)
- Add /api/reminders endpoint and cron send-reminders.ts script
- Add reminder fields to NewTaskModal and EditTaskModal components
- Fix reminder datetime serialization: use toISOString().slice(0,16) for
  UTC-safe YYYY-MM-DDTHH:mm format compatible with datetime-local inputs
- Update Dockerfile to install tsx for cron container
- Add AGENTS.md with project conventions
- Update docker-compose.yml with cron service and build context

Dockerfile

@@ -43,6 +43,12 @@ COPY --from=builder /app/prisma ./prisma
 # Copy generated Prisma client (includes native query engine binary)
 COPY --from=builder /app/src/generated ./src/generated
 
+# Copy cron script for reminder notifications
+COPY --from=builder /app/scripts ./scripts
+
+# Install tsx for cron worker
+RUN npm install -g tsx
+
 # Copy startup script
 COPY start.sh /app/start.sh
 RUN chmod +x /app/start.sh