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_review_proofs.py

@@ -14,6 +14,7 @@ each precondition of a blind queue review:
 
 These are the harness assertions from the issue's Required behavior 7.
 """
+import io
 import sys
 import unittest
 
@@ -463,5 +464,30 @@ class TestFinalReport(unittest.TestCase):
         self.assertFalse(report["merge_allowed"])
 
 
+class TestStdoutIsolation(unittest.TestCase):
+    """Regression test for #178: tests must not close or corrupt stdout/stderr
+    (prevents need for junitxml workaround in full suite runs and review validation).
+    """
+
+    def test_stdout_remains_usable(self):
+        """After typical test activity (mocks, redirects, prints from mains), stdout should be usable."""
+        # Verify not closed
+        self.assertFalse(getattr(sys.stdout, "closed", False))
+        # Should be able to write (even if captured by pytest)
+        try:
+            sys.stdout.write("")
+            sys.stdout.flush()
+        except Exception as exc:
+            self.fail(f"stdout write failed after test activity: {exc}")
+
+    def test_stderr_remains_usable(self):
+        self.assertFalse(getattr(sys.stderr, "closed", False))
+        try:
+            sys.stderr.write("")
+            sys.stderr.flush()
+        except Exception as exc:
+            self.fail(f"stderr write failed: {exc}")
+
+
 if __name__ == "__main__":
     unittest.main()