test: isolate CLI output capture with monkeypatch/redirect to fix stdout corruption in full suite runs (Issue #178)

- Replace raw patch("sys.stdout") and contextlib.redirect_stderr with pytest MonkeyPatch or contextlib redirect scoped to main calls in CLI tests (create_*, prs, python_cli, manage_labels, merge_pr, review_pr).
- Add regression tests in test_review_proofs.py for stdout/stderr remaining usable.
- Ensures normal pytest summary output without junitxml workaround; improves review validation reporting per #173.
- No behavior change to production code or gates.

Refs #178

tests/test_merge_pr.py

@@ -45,13 +45,17 @@ class TestMergeDisabled(unittest.TestCase):
         mock_api.assert_not_called()
 
     def test_message_points_to_gated_workflow(self):
+        from _pytest.monkeypatch import MonkeyPatch
         import io
-        import contextlib
         with patch("merge_pr.get_auth_header", return_value=FAKE_CREDS), \
                 patch("merge_pr.api_request") as mock_api:
             buf = io.StringIO()
-            with contextlib.redirect_stderr(buf):
+            monkeypatch = MonkeyPatch()
+            monkeypatch.setattr(sys, "stderr", buf)
+            try:
                 rc = merge_pr.main(["--pr-number", "81"])
+            finally:
+                monkeypatch.undo()
         self.assertEqual(rc, 2)
         mock_api.assert_not_called()
         msg = buf.getvalue().lower()