fix(cron): add service-layer timezone validation

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>

nanobot/cli/commands.py

@@ -787,15 +787,19 @@ def cron_add(
     store_path = get_data_dir() / "cron" / "jobs.json"
     service = CronService(store_path)
     
-    job = service.add_job(
-        name=name,
-        schedule=schedule,
-        message=message,
-        deliver=deliver,
-        to=to,
-        channel=channel,
-    )
-    
+    try:
+        job = service.add_job(
+            name=name,
+            schedule=schedule,
+            message=message,
+            deliver=deliver,
+            to=to,
+            channel=channel,
+        )
+    except ValueError as e:
+        console.print(f"[red]Error: {e}[/red]")
+        raise typer.Exit(1) from e
+
     console.print(f"[green]✓[/green] Added job '{job.name}' ({job.id})")