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_core_will_invalidate.py

@@ -22,15 +22,14 @@ import os
 import sys
 from unittest.mock import MagicMock, patch
 
-import pytest
-
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
-from bal.core.will import Will, WillItem
-
 # Patch Transaction.add_info_from_wallet so WillItem can parse the tx hex
 # without a live Electrum wallet connection.
 from electrum.transaction import Transaction
+
+from bal.core.will import Will, WillItem
+
 _patcher = patch.object(Transaction, "add_info_from_wallet")
 _patcher.start()