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>
