feat: 新增节假日缓存，仅工作日生成与推送

- daily/holiday.py: 抓取 xiaoai.me 全年节假日并缓存到 .cache/holidays-<year>.json，
  is_workday() 判定（法定节假日/周末休息，调休补班日上班），联网失败回退本地缓存
- scheduler: tick_once 前置工作日闸门，非工作日标记完成并跳过，判定失败降级为正常工作日避免漏跑
- config/.env.example: 新增 DAILY_WORKDAY_ONLY 开关（默认开）
- tests: test_holiday 8 例 + scheduler 非工作日闸门 2 例

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_holiday.py

@@ -0,0 +1,57 @@
+"""Tests for daily.holiday."""
+
+from __future__ import annotations
+
+import unittest
+from datetime import date
+
+from daily.holiday import is_workday, workday_name
+
+# 模拟 2026 节假日表：1/1 元旦(休)，1/4 元旦调休(补班)
+HOLIDAYS = {
+    "2026-01-01": {"rest": True, "name": "元旦节"},
+    "2026-01-02": {"rest": True, "name": "元旦节"},
+    "2026-01-03": {"rest": True, "name": "元旦节"},
+    "2026-01-04": {"rest": False, "name": "元旦节调休"},
+    "2026-02-16": {"rest": True, "name": "春节"},
+    "2026-02-14": {"rest": False, "name": "春节调休"},
+}
+
+
+class IsWorkdayTests(unittest.TestCase):
+    def test_legal_holiday_is_rest(self):
+        # 2026-01-01 周四，元旦 → 休息
+        self.assertFalse(is_workday(date(2026, 1, 1), HOLIDAYS))
+
+    def test_tiaoxiu_makeup_day_is_workday(self):
+        # 2026-01-04 周日，调休补班 → 上班
+        self.assertEqual(date(2026, 1, 4).weekday(), 6)
+        self.assertTrue(is_workday(date(2026, 1, 4), HOLIDAYS))
+
+    def test_spring_festival_holiday(self):
+        self.assertFalse(is_workday(date(2026, 2, 16), HOLIDAYS))
+
+    def test_spring_festival_makeup_saturday(self):
+        # 2026-02-14 周六，调休 → 上班
+        self.assertEqual(date(2026, 2, 14).weekday(), 5)
+        self.assertTrue(is_workday(date(2026, 2, 14), HOLIDAYS))
+
+    def test_normal_weekday(self):
+        # 2026-07-23 周四，非节假日 → 上班
+        self.assertTrue(is_workday(date(2026, 7, 23), HOLIDAYS))
+
+    def test_normal_weekend(self):
+        # 2026-07-25 周六，非节假日 → 休息
+        self.assertFalse(is_workday(date(2026, 7, 25), HOLIDAYS))
+
+
+class WorkdayNameTests(unittest.TestCase):
+    def test_holiday_name(self):
+        self.assertEqual(workday_name(date(2026, 1, 1), HOLIDAYS), "元旦节")
+
+    def test_normal_day_has_no_name(self):
+        self.assertIsNone(workday_name(date(2026, 7, 23), HOLIDAYS))
+
+
+if __name__ == "__main__":
+    unittest.main()