refactor(ui): merge the one-expression converters into SimpleConverters.cs

Six files of three lines each. NotNullToBool is gone entirely — Avalonia
ships ObjectConverters.IsNotNull, so TaskRowView binds that directly.

src/ClaudeDo.App/App.axaml

@@ -13,7 +13,6 @@
             </ResourceDictionary.MergedDictionaries>
 
             <!-- Converters -->
-            <converters:NotNullToBoolConverter x:Key="NotNullToBool"/>
             <converters:StrikeIfTrueConverter  x:Key="StrikeIfTrue"/>
             <converters:EqStatusConverter      x:Key="EqStatus"/>
             <converters:UpperCaseConverter     x:Key="UpperCase"/>