fix(config): install timezone data on all platforms

tests/config/test_model_presets.py

@@ -1,4 +1,8 @@
 import json
+import os
+import subprocess
+import sys
+import textwrap
 import warnings
 
 import pytest
@@ -42,6 +46,32 @@ def test_agent_timezone_rejects_unknown_iana_name() -> None:
         Config.model_validate({"agents": {"defaults": {"timezone": "Not/AZone"}}})
 
 
+def test_agent_timezones_use_packaged_data_without_system_database() -> None:
+    script = textwrap.dedent(
+        """\
+        from zoneinfo import TZPATH
+
+        from nanobot.config.schema import Config
+
+        assert not TZPATH
+        for name in ("UTC", "Asia/Shanghai"):
+            config = Config.model_validate({"agents": {"defaults": {"timezone": name}}})
+            serialized = config.model_dump(mode="json", by_alias=True)
+            restored = Config.model_validate(serialized)
+            assert restored.agents.defaults.timezone == name
+        """
+    )
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        env=os.environ | {"PYTHONTZPATH": ""},
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+
+    assert result.returncode == 0, result.stderr
+
+
 def test_provider_api_type_accepts_exact_values_only() -> None:
     config = Config.model_validate({
         "providers": {