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_anticipate_manual_locktime.py

@@ -27,19 +27,19 @@ Run:
         tests/test_anticipate_manual_locktime.py -q
 """
 
-import sys
-import os
 import copy
+import os
+import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
 import pytest  # noqa: E402
 
 from bal.core.will import (  # noqa: E402
-    WillItem,
-    Will,
     NotCompleteWillException,
+    Will,
     WillExpiredException,
+    WillItem,
 )
 
 # A valid serialized tx (1 input + 1 output, version 2).