lint: ruff cleanup pass across bal/ and tests/

- Sort imports and fix pyproject ruff config (per-file ignores for
  intentional Qt/core exceptions)
- Mark Heirs.validate_* helpers as @staticmethod
- Clean up dead code, rename shadowing vars, use raise ... from
- Add AGENTS.md with env/lint/test/release guidance

tests/preview_building_will_close_btn.py

@@ -17,10 +17,15 @@ import os
 
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
-from PyQt6.QtWidgets import (  # noqa: E402
-    QApplication, QWidget, QVBoxLayout, QHBoxLayout, QLabel, QPushButton,
-)
 from PyQt6.QtCore import Qt  # noqa: E402
+from PyQt6.QtWidgets import (  # noqa: E402
+    QApplication,
+    QHBoxLayout,
+    QLabel,
+    QPushButton,
+    QVBoxLayout,
+    QWidget,
+)
 
 COLOR_OK = "#05ad05"