fix(background-agent): omit model field to use session's lastModel

Previously, background task completion notifications passed parentModel when defined, causing OpenCode to use default Sonnet model when parentModel was undefined. Now model field is always omitted, letting OpenCode use the session's existing lastModel (like todo-continuation hook).

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/manager.ts

@@ -639,16 +639,14 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     }
 
     // Inject notification via session.prompt with noReply
-    // Preserve parent session's model/agent context to prevent model switching
+    // Don't pass model - let OpenCode use session's existing lastModel (like todo-continuation)
+    // This prevents model switching when parentModel is undefined
     try {
       await this.client.session.prompt({
         path: { id: task.parentSessionID },
         body: {
           noReply: !allComplete,
           ...(task.parentAgent !== undefined ? { agent: task.parentAgent } : {}),
-          ...(task.parentModel?.providerID && task.parentModel?.modelID
-            ? { model: { providerID: task.parentModel.providerID, modelID: task.parentModel.modelID } }
-            : {}),
           parts: [{ type: "text", text: notification }],
         },
       })