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_service.py

@@ -0,0 +1,30 @@
+import pytest
+
+from nanobot.cron.service import CronService
+from nanobot.cron.types import CronSchedule
+
+
+def test_add_job_rejects_unknown_timezone(tmp_path) -> None:
+    service = CronService(tmp_path / "cron" / "jobs.json")
+
+    with pytest.raises(ValueError, match="unknown timezone 'America/Vancovuer'"):
+        service.add_job(
+            name="tz typo",
+            schedule=CronSchedule(kind="cron", expr="0 9 * * *", tz="America/Vancovuer"),
+            message="hello",
+        )
+
+    assert service.list_jobs(include_disabled=True) == []
+
+
+def test_add_job_accepts_valid_timezone(tmp_path) -> None:
+    service = CronService(tmp_path / "cron" / "jobs.json")
+
+    job = service.add_job(
+        name="tz ok",
+        schedule=CronSchedule(kind="cron", expr="0 9 * * *", tz="America/Vancouver"),
+        message="hello",
+    )
+
+    assert job.schedule.tz == "America/Vancouver"
+    assert job.state.next_run_at_ms is not None