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/test_gui_widgets.py

@@ -13,13 +13,11 @@ Run:
 """
 
 import sys
+
 sys.path.insert(0, __file__.rsplit("/", 2)[0])
 
-from PyQt6.QtCore import QTimer
 from PyQt6.QtWidgets import QApplication, QWidget
 
-from electrum.util import DECIMAL_POINT, decimal_point_to_base_unit_name
-
 _app = QApplication.instance() or QApplication(sys.argv)