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_group_i_basic_checkalive.py

@@ -32,7 +32,6 @@ sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
 from bal.gui.qt.window import BalWindow  # noqa: E402  (path insert above)
 
-
 OFFSET = BalWindow.BASIC_MODE_CHECK_ALIVE_OFFSET_SECONDS