Fix combined conversation's model to reflect sources, not the merge model

primaryModel was being set to the model that performed the merge (or,
in AI mode, stamped onto every synthesized message). It should instead
be the most recently used model among the source conversations being
combined.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Services/ConversationMergeService.swift

@@ -67,6 +67,14 @@ enum ConversationMergeService {
             try DatabaseService.shared.loadConversation(id: id)
         }
 
+        // The model used in the merged conversation should reflect the most recently used
+        // model across the *source* conversations — never the model that performed the merge.
+        let latestModelId = sources
+            .flatMap { $0.1 }
+            .filter { $0.modelId != nil }
+            .max { $0.timestamp < $1.timestamp }?
+            .modelId
+
         let mergedMessages: [Message]
         switch mode {
         case .simple:
@@ -75,7 +83,12 @@ enum ConversationMergeService {
             mergedMessages = try await aiMerge(sources)
         }
 
-        let newConversation = try DatabaseService.shared.saveConversation(name: name, messages: mergedMessages)
+        let newConversation = try DatabaseService.shared.saveConversation(
+            id: UUID(),
+            name: name,
+            messages: mergedMessages,
+            primaryModel: latestModelId
+        )
 
         if deleteOriginals {
             for id in conversationIds {
@@ -148,13 +161,15 @@ enum ConversationMergeService {
 
         let turns = try parseTurns(from: response.content)
 
+        // modelId intentionally left nil here: these messages are a synthesized composite,
+        // not output from a single source model. The conversation's primaryModel (set by the
+        // caller from the source conversations) is what drives the model shown in the list.
         let base = Date()
         return turns.enumerated().map { index, turn in
             Message(
                 role: turn.role == "user" ? .user : .assistant,
                 content: turn.content,
-                timestamp: base.addingTimeInterval(TimeInterval(index)),
+                timestamp: base.addingTimeInterval(TimeInterval(index))
-                modelId: modelId
             )
         }
     }