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_extra.py

@@ -8,15 +8,16 @@ Run:
     QT_QPA_PLATFORM=offscreen python3 tests/test_core_will_extra.py
 """
 
-import sys
 import os
-from unittest.mock import MagicMock, patch, PropertyMock, call
+import sys
+from unittest.mock import MagicMock, patch
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
-from bal.core.will import Will, WillItem
 from electrum.transaction import Transaction
 
+from bal.core.will import Will, WillItem
+
 _VALID_TX_HEX = (
     "01000000012a5c9a94fcde98f5581cd00162c60a13936ceb75389ea65b"
     "f38633b424eb4031000000006c493046022100a82bbc57a0136751e543"