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_f_heir_change_rebuild.py

@@ -16,8 +16,9 @@ Run:
     python3 tests/test_group_f_heir_change_rebuild.py
 """
 
-import sys
 import os
+import sys
+
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
 from bal.core.will import Will