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_g_basic_calendar.py

@@ -24,8 +24,7 @@ import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), os.pardir))
 
-from bal.gui.qt.widgets import (BASIC_REMINDER_OFFSETS,
-                                 basic_reminder_offsets)
+from bal.gui.qt.widgets import BASIC_REMINDER_OFFSETS, basic_reminder_offsets
 
 
 def test_basic_offsets_all_future():