fix(ui): suppress auto-open conflict resolver during MCP-driven merges

review_task/continue_merge on a planning parent always leaves conflicts in
the tree, and the UI auto-opened the resolver on every PlanningMergeConflict
broadcast regardless of who started the merge -- so a running Claude session
resolving a unit-merge conflict could race a human editing the same shared
checkout in a resolver window neither of them asked for.

PlanningMergeOrchestrator.StartAsync now takes an externallyDriven flag (set
by ExternalMcpService's MCP-driven review_task path, left false for the UI's
ApproveReview) that rides along on the PlanningMergeConflict broadcast. The
UI only auto-opens the resolver when it's false; otherwise it shows a
persistent banner with a manual "Open resolver" button, cleared on
PlanningMergeAborted/PlanningCompleted. A new GetActiveExternalConflictsAsync
query (checked against GitService.IsMidMergeAsync rather than the in-memory
flag alone) lets the UI resync the banner on reconnect instead of trusting a
one-shot broadcast that isn't replayed after a restart.

The childless single-task conflict path was checked and needed no change --
it only broadcasts the generic TaskUpdated, never PlanningMergeConflict.

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -258,5 +258,27 @@
       </StackPanel>
     </Border>
 
+    <!-- External merge popup card — bottom-left corner, while an MCP session owns an
+         unresolved unit-merge conflict. Does not auto-dismiss; stays until the merge ends. -->
+    <Border Grid.Row="0" Grid.RowSpan="3"
+            VerticalAlignment="Bottom" HorizontalAlignment="Left"
+            Margin="16,0,0,30"
+            ZIndex="100"
+            IsVisible="{Binding IsExternalMergeBannerVisible}"
+            Background="{DynamicResource DeepBrush}"
+            BorderBrush="{DynamicResource PeatBrush}"
+            BorderThickness="1"
+            CornerRadius="6"
+            Padding="14,10">
+      <StackPanel Spacing="8" Width="300">
+        <TextBlock Classes="body" Text="{loc:Tr shell.externalMerge.banner}" TextWrapping="Wrap"/>
+        <StackPanel Orientation="Horizontal" Spacing="8" HorizontalAlignment="Right">
+          <Button Classes="btn"
+                  Content="{loc:Tr shell.externalMerge.open}"
+                  Command="{Binding OpenExternalMergeConflictCommand}"/>
+        </StackPanel>
+      </StackPanel>
+    </Border>
+
   </Grid>
 </Window>