test: cover atomic temp cleanup on interrupts

- add regression coverage for BaseException cleanup in atomic_json_write
- add dedicated atomic_yaml_write tests, including interrupt cleanup
- document why BaseException is intentional in both helpers

utils.py

@@ -50,6 +50,8 @@ def atomic_json_write(
             os.fsync(f.fileno())
         os.replace(tmp_path, path)
     except BaseException:
+        # Intentionally catch BaseException so temp-file cleanup still runs for
+        # KeyboardInterrupt/SystemExit before re-raising the original signal.
         try:
             os.unlink(tmp_path)
         except OSError:
@@ -96,6 +98,8 @@ def atomic_yaml_write(
             os.fsync(f.fileno())
         os.replace(tmp_path, path)
     except BaseException:
+        # Match atomic_json_write: cleanup must also happen for process-level
+        # interruptions before we re-raise them.
         try:
             os.unlink(tmp_path)
         except OSError: