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_theme.py

@@ -8,11 +8,11 @@ Run:
 """
 
 import sys
+
 sys.path.insert(0, __file__.rsplit("/", 2)[0])
 
 from bal.gui.qt.theme import status_color
 
-
 # ------------------------------------------------------------------ #
 # Helpers
 # ------------------------------------------------------------------ #