Adds `_validate_schedule_for_add()` to `CronService.add_job` so that
invalid or misplaced `tz` values are rejected before a job is persisted,
regardless of which caller (CLI, tool, etc.) invoked the service.
Surfaces the resulting `ValueError` in `nanobot cron add` via a
`try/except` so the CLI exits cleanly with a readable error message.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
With tz: Use the specified timezone (e.g., "Asia/Shanghai").
Without tz: Use the local timezone (datetime.now().astimezone().tzinfo) instead of defaulting to UTC
When schedule.tz is present, use the specified timezone to calculate the next execution time, ensuring scheduled tasks trigger correctly across different timezones.
