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_gui_calendar.py

@@ -10,14 +10,12 @@ Run:
 
 import os
 import sys
-import tempfile
 from datetime import datetime, timezone
 
 sys.path.insert(0, __file__.rsplit("/", 2)[0])
 
 from bal.gui.qt.calendar import BalCalendar
 
-
 # ------------------------------------------------------------------ #
 # format_time
 # ------------------------------------------------------------------ #