fix(run): _should_skip tz check uses isinstance, tolerates mock cfg

Existing lifecycle tests mock cfg via MagicMock; the attribute auto-return
made `cfg.operating_hours._tz_cache` truthy-but-not-a-tzinfo, crashing
datetime.fromtimestamp with TypeError. Guard with isinstance(tz, tzinfo)
so mock configs are silently skipped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/atm/main.py

@@ -8,7 +8,7 @@ import os
 import sys
 import time
 from dataclasses import dataclass
-from datetime import datetime
+from datetime import datetime, tzinfo
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Callable, Protocol, cast
 
@@ -742,8 +742,8 @@ def _should_skip(now_ts: float, state: LifecycleState, cfg, canary) -> str | Non
     if oh is None or not oh.enabled:
         return None
     tz = getattr(oh, "_tz_cache", None)
-    if tz is None:
-        # Enabled but no tz resolved — skip the check rather than crash mid-loop.
+    if not isinstance(tz, tzinfo):
+        # Enabled but no tz resolved (or mock cfg in tests) — skip rather than crash.
         return None
     now_exchange = datetime.fromtimestamp(now_ts, tz=tz)
     # weekday() = 0..6 (MON..SUN). Locale-free; strftime('%a') is not.