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_build_will_dialog.py

@@ -17,8 +17,8 @@ import sys
 
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
-from PyQt6.QtWidgets import QApplication, QLabel, QWidget, QVBoxLayout
 from PyQt6.QtCore import Qt
+from PyQt6.QtWidgets import QApplication, QLabel, QVBoxLayout, QWidget
 
 # Same colors as BalBuildWillDialog
 COLOR_WARNING = "#cfa808"