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>

tests/test_cron_commands.py

@@ -0,0 +1,29 @@
+from typer.testing import CliRunner
+
+from nanobot.cli.commands import app
+
+runner = CliRunner()
+
+
+def test_cron_add_rejects_invalid_timezone(monkeypatch, tmp_path) -> None:
+    monkeypatch.setattr("nanobot.config.loader.get_data_dir", lambda: tmp_path)
+
+    result = runner.invoke(
+        app,
+        [
+            "cron",
+            "add",
+            "--name",
+            "demo",
+            "--message",
+            "hello",
+            "--cron",
+            "0 9 * * *",
+            "--tz",
+            "America/Vancovuer",
+        ],
+    )
+
+    assert result.exit_code == 1
+    assert "Error: unknown timezone 'America/Vancovuer'" in result.stdout
+    assert not (tmp_path / "cron" / "jobs.json").exists()