fix(desktop): keep sidebar replies at current layer

Stop sending sidebar replies through the agent reply-parent override path and keep same-branch replies visually flat so plain in-thread sends stay at the current drilled level.

Made-with: Cursor

desktop/src/features/channels/ui/ChannelScreen.tsx

@@ -397,11 +397,9 @@ export function ChannelScreen({
       if (!parentEventId) {
         return;
       }
-      const agentReplyParentId = currentThreadHeadId ?? null;
       const branchHeadId = currentThreadHeadId ?? null;
 
       await sendMessageMutation.mutateAsync({
-        agentReplyParentId,
         branchHeadId,
         content,
         mentionPubkeys,

desktop/src/features/messages/lib/threadBranch.ts

@@ -74,7 +74,12 @@ export function collectThreadBranch(
     )
     .map((message) => ({
       ...message,
-      depth: Math.max(0, message.depth - depthOffset),
+      // Keep normal replies in the drilled branch visually flat.
+      // Only explicitly drilled nested branches get their own branchHeadId and collapse link.
+      depth:
+        message.branchHeadId === branchHeadId
+          ? 0
+          : Math.max(0, message.depth - depthOffset),
     }));
 
   return {