feat(worker): clamp max-turns to a configurable ceiling

Runaway sessions were the single biggest cost driver: model_presets was
never persisted (stayed code-only), default_max_turns shipped at 100, and
ResolveMaxTurns had no upper bound, so a task/list override could run
hundreds of turns unchecked.

- TaskRunner.ResolveMaxTurns now clamps the resolved value to
  AppSettings.MaxTurnsCeiling (new column, default 80) and logs a warning
  with task id / requested / effective value when it clamps.
- default_max_turns default lowered from 100 to 40 (entity, EF config,
  and the seeded row via the new AddMaxTurnsCeiling migration).
- AppSettingsRepository.GetAsync backfills model_presets with the
  shipping defaults on first read instead of leaving the column null.
- Settings > General's per-model preset table and the task/list agent
  editor now show a hint when a set max-turns value exceeds the ceiling.

src/ClaudeDo.Ui/Views/Controls/AgentConfigEditor.axaml

@@ -34,6 +34,9 @@
                      PlaceholderText="{Binding TurnsInheritedHint}"
                      Minimum="1" Maximum="200" Increment="1" FormatString="0"
                      HorizontalAlignment="Stretch"/>
+      <TextBlock Classes="meta" Opacity="0.6" TextWrapping="Wrap"
+                 Text="{Binding TurnsCeilingHint}"
+                 IsVisible="{Binding TurnsCeilingHint, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
     </StackPanel>
 
     <!-- System prompt -->

src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml

@@ -129,14 +129,19 @@
                 <ItemsControl ItemsSource="{Binding General.ModelPresets}">
                   <ItemsControl.ItemTemplate>
                     <DataTemplate x:DataType="settings:ModelPresetRowViewModel">
-                      <Grid ColumnDefinitions="90,12,*,12,110" Margin="0,0,0,6">
-                        <TextBlock Grid.Column="0" Text="{Binding Model}" VerticalAlignment="Center"/>
-                        <ComboBox Grid.Column="2" ItemsSource="{Binding EffortLevels}"
-                                  SelectedItem="{Binding Effort, Mode=TwoWay}"
-                                  HorizontalAlignment="Stretch"/>
-                        <NumericUpDown Grid.Column="4" Value="{Binding MaxTurns, Mode=TwoWay}"
-                                       Minimum="1" Maximum="200" Increment="1" FormatString="0"/>
-                      </Grid>
+                      <StackPanel Spacing="2" Margin="0,0,0,6">
+                        <Grid ColumnDefinitions="90,12,*,12,110">
+                          <TextBlock Grid.Column="0" Text="{Binding Model}" VerticalAlignment="Center"/>
+                          <ComboBox Grid.Column="2" ItemsSource="{Binding EffortLevels}"
+                                    SelectedItem="{Binding Effort, Mode=TwoWay}"
+                                    HorizontalAlignment="Stretch"/>
+                          <NumericUpDown Grid.Column="4" Value="{Binding MaxTurns, Mode=TwoWay}"
+                                         Minimum="1" Maximum="200" Increment="1" FormatString="0"/>
+                        </Grid>
+                        <TextBlock Classes="meta" Opacity="0.6" TextWrapping="Wrap"
+                                   Text="{Binding ClampHint}"
+                                   IsVisible="{Binding ClampHint, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                      </StackPanel>
                     </DataTemplate>
                   </ItemsControl.ItemTemplate>
                 </ItemsControl>