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_past_locktime.py

@@ -22,13 +22,13 @@ whether a fix is needed.  Run:
         python3 -m pytest tests/test_anticipate_past_locktime.py -q
 """
 
-import sys
 import os
+import sys
 import time
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
-from bal.core.util import Util, LOCKTIME_THRESHOLD
+from bal.core.util import LOCKTIME_THRESHOLD, Util
 
 
 # ---------------------------------------------------------------------------