fix(consistency): correct behind-remote detection to not trigger on local-ahead

_detect_behind_remote was comparing HEAD != @{u} which incorrectly
triggered C-16 when the local repo had unpushed commits. Fixed to use
git rev-list --count HEAD..@{u} which only counts commits the remote
has that local lacks. Adds test_returns_false_when_local_ahead.

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

tests/test_consistency_check.py

@@ -450,6 +450,26 @@ class TestDetectBehindRemote:
         subprocess.run(["git", "-C", str(clone), "push", "origin", "HEAD"], capture_output=True)
         assert _detect_behind_remote(str(clone)) is False
 
+    def test_returns_false_when_local_ahead(self, tmp_path):
+        """Local has unpushed commits — NOT considered behind."""
+        import subprocess
+        bare = tmp_path / "bare.git"
+        clone = tmp_path / "clone"
+        bare.mkdir()
+        subprocess.run(["git", "init", "--bare", str(bare)], capture_output=True)
+        subprocess.run(["git", "clone", str(bare), str(clone)], capture_output=True)
+        subprocess.run(
+            ["git", "-C", str(clone), "commit", "--allow-empty", "-m", "init"],
+            capture_output=True,
+        )
+        subprocess.run(["git", "-C", str(clone), "push", "origin", "HEAD:main"], capture_output=True)
+        # Add a local-only commit (not pushed)
+        subprocess.run(
+            ["git", "-C", str(clone), "commit", "--allow-empty", "-m", "local only"],
+            capture_output=True,
+        )
+        assert _detect_behind_remote(str(clone)) is False
+
     def test_returns_true_when_behind(self, tmp_path):
         """Remote has a commit the clone doesn't → clone is behind."""
         import subprocess