fix(background-agent): honor explicit model override in manager

Keep BackgroundManager launch and resume from sending both agent and model so OpenCode does not override configured subagent models. Add launch and resume regressions for the live production path.

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

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

src/features/background-agent/manager.test.ts

@@ -1668,7 +1668,7 @@ describe("BackgroundManager.resume model persistence", () => {
     // then - model should be passed in prompt body
     expect(promptCalls).toHaveLength(1)
     expect(promptCalls[0].body.model).toEqual({ providerID: "anthropic", modelID: "claude-sonnet-4-20250514" })
-    expect(promptCalls[0].body.agent).toBe("explore")
+    expect("agent" in promptCalls[0].body).toBe(false)
   })
 
   test("should NOT pass model when task has no model (backward compatibility)", async () => {
@@ -1806,9 +1806,9 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
       expect(task.sessionID).toBeUndefined()
     })
 
-    test("should return immediately even with concurrency limit", async () => {
-      // given
-      const config = { defaultConcurrency: 1 }
+  test("should return immediately even with concurrency limit", async () => {
+    // given
+    const config = { defaultConcurrency: 1 }
       manager.shutdown()
       manager = new BackgroundManager({ client: mockClient, directory: tmpdir() } as unknown as PluginInput, config)
 
@@ -1828,9 +1828,76 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
 
       // then
       expect(endTime - startTime).toBeLessThan(100) // Should be instant
-      expect(task1.status).toBe("pending")
-      expect(task2.status).toBe("pending")
+    expect(task1.status).toBe("pending")
+    expect(task2.status).toBe("pending")
+  })
+
+  test("should omit agent when launch has model and keep agent without model", async () => {
+    // given
+    const promptBodies: Array<Record<string, unknown>> = []
+    let resolveFirstPromptStarted: (() => void) | undefined
+    let resolveSecondPromptStarted: (() => void) | undefined
+    const firstPromptStarted = new Promise<void>((resolve) => {
+      resolveFirstPromptStarted = resolve
     })
+    const secondPromptStarted = new Promise<void>((resolve) => {
+      resolveSecondPromptStarted = resolve
+    })
+    const customClient = {
+      session: {
+        create: async (_args?: unknown) => ({ data: { id: `ses_${crypto.randomUUID()}` } }),
+        get: async () => ({ data: { directory: "/test/dir" } }),
+        prompt: async () => ({}),
+        promptAsync: async (args: { path: { id: string }; body: Record<string, unknown> }) => {
+          promptBodies.push(args.body)
+          if (promptBodies.length === 1) {
+            resolveFirstPromptStarted?.()
+          }
+          if (promptBodies.length === 2) {
+            resolveSecondPromptStarted?.()
+          }
+          return {}
+        },
+        messages: async () => ({ data: [] }),
+        todo: async () => ({ data: [] }),
+        status: async () => ({ data: {} }),
+        abort: async () => ({}),
+      },
+    }
+    manager.shutdown()
+    manager = new BackgroundManager({ client: customClient, directory: tmpdir() } as unknown as PluginInput)
+
+    const launchInputWithModel = {
+      description: "Test task with model",
+      prompt: "Do something",
+      agent: "test-agent",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+    }
+    const launchInputWithoutModel = {
+      description: "Test task without model",
+      prompt: "Do something else",
+      agent: "test-agent",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+    }
+
+    // when
+    const taskWithModel = await manager.launch(launchInputWithModel)
+    await firstPromptStarted
+    const taskWithoutModel = await manager.launch(launchInputWithoutModel)
+    await secondPromptStarted
+
+    // then
+    expect(taskWithModel.status).toBe("pending")
+    expect(taskWithoutModel.status).toBe("pending")
+    expect(promptBodies).toHaveLength(2)
+    expect(promptBodies[0].model).toEqual({ providerID: "anthropic", modelID: "claude-opus-4-6" })
+    expect("agent" in promptBodies[0]).toBe(false)
+    expect(promptBodies[1].agent).toBe("test-agent")
+    expect("model" in promptBodies[1]).toBe(false)
+  })
 
     test("should queue multiple tasks without blocking", async () => {
       // given

src/features/background-agent/manager.ts

@@ -515,7 +515,9 @@ export class BackgroundManager {
     promptWithModelSuggestionRetry(this.client, {
       path: { id: sessionID },
       body: {
-        agent: input.agent,
+        // When a model is explicitly provided, omit the agent name so opencode's
+        // built-in agent fallback chain does not override the user-specified model.
+        ...(launchModel ? {} : { agent: input.agent }),
         ...(launchModel ? { model: launchModel } : {}),
         ...(launchVariant ? { variant: launchVariant } : {}),
         system: input.skillContent,
@@ -792,7 +794,9 @@ export class BackgroundManager {
     this.client.session.promptAsync({
       path: { id: existingTask.sessionID },
       body: {
-        agent: existingTask.agent,
+        // When a model is explicitly provided, omit the agent name so opencode's
+        // built-in agent fallback chain does not override the user-specified model.
+        ...(resumeModel ? {} : { agent: existingTask.agent }),
         ...(resumeModel ? { model: resumeModel } : {}),
         ...(resumeVariant ? { variant: resumeVariant } : {}),
         tools: (() => {