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_b_auto_sign.py

@@ -28,7 +28,6 @@ sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 from bal.core.plugin_base import BalConfig
 from bal.core.willexecutors import Willexecutors
 
-
 # ------------------------------------------------------------------ #
 # Mocks
 # ------------------------------------------------------------------ #