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_common.py

@@ -8,12 +8,13 @@ Run:
 """
 
 import sys
+
 sys.path.insert(0, __file__.rsplit("/", 2)[0])
 
-from PyQt6.QtWidgets import QApplication, QGridLayout, QLabel, QWidget
+from PyQt6.QtWidgets import QApplication, QGridLayout, QLabel
 
 # Import the module itself, not via "from .common import *"
-import bal.gui.qt.common as C
+import bal.gui.qt.common as common
 
 _app = QApplication.instance() or QApplication(sys.argv)
 
@@ -23,18 +24,18 @@ _app = QApplication.instance() or QApplication(sys.argv)
 # ------------------------------------------------------------------ #
 
 def test_shown_cv_default():
-    cv = C.shown_cv(True)
+    cv = common.shown_cv(True)
     assert cv.get() is True
 
 
 def test_shown_cv_set():
-    cv = C.shown_cv(True)
+    cv = common.shown_cv(True)
     cv.set(False)
     assert cv.get() is False
 
 
 def test_shown_cv_roundtrip():
-    cv = C.shown_cv(False)
+    cv = common.shown_cv(False)
     assert cv.get() is False
     cv.set(True)
     assert cv.get() is True
@@ -47,19 +48,19 @@ def test_shown_cv_roundtrip():
 # ------------------------------------------------------------------ #
 
 def test_check_alive_error_default():
-    err = C.CheckAliveError(1000000)
+    err = common.CheckAliveError(1000000)
     assert err.timestamp_to_check == 1000000
 
 
 def test_check_alive_error_str():
-    err = C.CheckAliveError(1000000)
+    err = common.CheckAliveError(1000000)
     s = str(err)
     assert "Check alive expired" in s
     assert "1970" in s
 
 
 def test_check_alive_error_subclass():
-    assert issubclass(C.CheckAliveError, Exception)
+    assert issubclass(common.CheckAliveError, Exception)
 
 
 # ------------------------------------------------------------------ #
@@ -68,17 +69,15 @@ def test_check_alive_error_subclass():
 
 def test_add_widget():
     grid = QGridLayout()
-    parent = QWidget()
     label = QLabel("test")
-    C.add_widget(grid, "Label", label, 0, "Help text")
+    common.add_widget(grid, "Label", label, 0, "Help text")
     assert grid.count() == 3  # label + widget + help button
 
 
 def test_add_widget_multiple_rows():
     grid = QGridLayout()
-    parent = QWidget()
-    C.add_widget(grid, "A", QLabel("a"), 0, "help_a")
-    C.add_widget(grid, "B", QLabel("b"), 1, "help_b")
+    common.add_widget(grid, "A", QLabel("a"), 0, "help_a")
+    common.add_widget(grid, "B", QLabel("b"), 1, "help_b")
     assert grid.count() == 6
 
 
@@ -87,7 +86,7 @@ def test_add_widget_multiple_rows():
 # ------------------------------------------------------------------ #
 
 def test_log_error_no_window():
-    C.log_error((Exception, Exception("test"), None))
+    common.log_error((Exception, Exception("test"), None))
 
 
 # ------------------------------------------------------------------ #