fix: freeze datetime in test_recent_commit_scores_max_recency

tests/test_git_vitality.py

@@ -1,7 +1,7 @@
 import pytest
+from datetime import datetime, timezone
 from unittest.mock import MagicMock, patch
-from pathlib import Path
-from arbiter.git_vitality import score_git_vitality, GitVitalityReport
+from arbiter.git_vitality import score_git_vitality
 from arbiter.git_historian import CommitInfo
 
 @pytest.fixture
@@ -33,15 +33,22 @@ def test_single_committer_scores_low_bus_factor(tmp_path, mock_commits):
 
 def test_recent_commit_scores_max_recency(tmp_path, mock_commits):
     (tmp_path / ".git").mkdir()
-    # Today is 2026-04-19 (from session context)
-    # mock_commits[0] is from 2026-04-10 (9 days ago)
-    with patch("arbiter.git_vitality.walk_commits", return_value=mock_commits):
-        with patch("subprocess.run") as mock_run:
-            mock_run.return_value.returncode = 0
-            mock_run.return_value.stdout = ""
-            report = score_git_vitality(tmp_path)
-            assert report.days_since_commit <= 30
-            # 25 points for recency
+    # Freeze "now" at 2026-04-19 so days_since stays deterministic.
+    # datetime.datetime is a C extension, so we patch the module binding
+    # rather than the classmethod (setattr fails on C types).
+    frozen_now = datetime(2026, 4, 19, tzinfo=timezone.utc)
+    mock_dt = MagicMock()
+    mock_dt.now = lambda *_args, **_kwargs: frozen_now
+    mock_dt.fromisoformat = lambda ts: datetime.fromisoformat(ts)
+    mock_dt.timezone = timezone
+    with patch("arbiter.git_vitality.datetime", mock_dt):
+        with patch("arbiter.git_vitality.walk_commits", return_value=mock_commits):
+            with patch("subprocess.run") as mock_run:
+                mock_run.return_value.returncode = 0
+                mock_run.return_value.stdout = ""
+                report = score_git_vitality(tmp_path)
+                assert report.days_since_commit == 8  # Apr 10 10:00 -> Apr 19 00:00
+                assert report.score >= 25  # max recency points
 
 def test_semver_tags_detected(tmp_path, mock_commits):
     (tmp_path / ".git").mkdir()