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.test.ts

@@ -675,9 +675,9 @@ describe("LaunchInput.skillContent", () => {
   })
 })
 
-describe("BackgroundManager.notifyParentSession - model/agent context preservation", () => {
-  test("should include model field in session.prompt when parentModel is defined", async () => {
-    // #given
+describe("BackgroundManager.notifyParentSession - agent context preservation", () => {
+  test("should never pass model field - let OpenCode use session's lastModel", async () => {
+    // #given - task with parentModel defined
     const task: BackgroundTask = {
       id: "task-with-model",
       sessionID: "session-child",
@@ -696,8 +696,8 @@ describe("BackgroundManager.notifyParentSession - model/agent context preservati
     // #when
     const promptBody = buildNotificationPromptBody(task)
 
-    // #then - model MUST be included when parentModel is defined
-    expect(promptBody.model).toEqual({ providerID: "anthropic", modelID: "claude-opus-4-5" })
+    // #then - model MUST NOT be passed (OpenCode uses session's lastModel)
+    expect("model" in promptBody).toBe(false)
     expect(promptBody.agent).toBe("Sisyphus")
   })
 
@@ -721,9 +721,9 @@ describe("BackgroundManager.notifyParentSession - model/agent context preservati
     // #when
     const promptBody = buildNotificationPromptBody(task)
 
-    // #then
+    // #then - no agent, no model (let OpenCode handle)
     expect("agent" in promptBody).toBe(false)
-    expect(promptBody.model).toEqual({ providerID: "anthropic", modelID: "claude-opus" })
+    expect("model" in promptBody).toBe(false)
   })
 
   test("should include agent field when parentAgent is defined", async () => {
@@ -748,9 +748,10 @@ describe("BackgroundManager.notifyParentSession - model/agent context preservati
 
     // #then
     expect(promptBody.agent).toBe("Sisyphus")
+    expect("model" in promptBody).toBe(false)
   })
 
-  test("should not pass model field when parentModel is undefined", async () => {
+  test("should not pass model field even when parentModel is undefined", async () => {
     // #given
     const task: BackgroundTask = {
       id: "task-no-model",
@@ -770,7 +771,7 @@ describe("BackgroundManager.notifyParentSession - model/agent context preservati
     // #when
     const promptBody = buildNotificationPromptBody(task)
 
-    // #then
+    // #then - model never passed regardless of parentModel
     expect("model" in promptBody).toBe(false)
     expect(promptBody.agent).toBe("Sisyphus")
   })
@@ -785,9 +786,8 @@ function buildNotificationPromptBody(task: BackgroundTask): Record<string, unkno
     body.agent = task.parentAgent
   }
 
-  if (task.parentModel?.providerID && task.parentModel?.modelID) {
-    body.model = { providerID: task.parentModel.providerID, modelID: task.parentModel.modelID }
-  }
+  // Don't pass model - let OpenCode use session's existing lastModel
+  // This prevents model switching when parentModel is undefined or different
 
   return body
 }

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 }],
         },
       })