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.Worker/External/ExternalMcpService.cs

@@ -423,7 +423,10 @@ public sealed class ExternalMcpService
 
             if (hasChildren)
             {
-                await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken);
+                // externallyDriven: true — this call came from an MCP session, not the UI's
+                // Approve button. A unit-merge conflict must not auto-open the in-app resolver;
+                // the driving session resolves it via continue_merge/abort_merge instead.
+                await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken, externallyDriven: true);
                 var parentDone = (await _tasks.GetByIdAsync(taskId, cancellationToken))!.Status == TaskStatus.Done;
                 mergeStatus = parentDone ? TaskMergeService.StatusMerged : TaskMergeService.StatusConflict;
                 if (!parentDone)