fix(agent): always keep agent when model is explicitly configured

Previously, when an explicit model was configured, the agent name was
omitted to prevent opencode's built-in agent fallback chain from
overriding the user-specified model. This removes that conditional logic
and always passes the agent name alongside the model. Tests are updated
to reflect this behavior change.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

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

src/tools/delegate-task/sync-prompt-sender.test.ts

@@ -165,6 +165,55 @@ bunDescribe("sendSyncPrompt", () => {
     bunExpect(promptArgs.body.tools.call_omo_agent).toBe(true)
   })
 
+  bunTest("includes agent alongside explicit category model", async () => {
+    //#given
+    const { sendSyncPrompt } = require("./sync-prompt-sender")
+
+    let promptArgs: any
+    const promptAsync = bunMock(async (input: any) => {
+      promptArgs = input
+      return { data: {} }
+    })
+
+    const mockClient = {
+      session: {
+        promptAsync,
+      },
+    }
+
+    const input = {
+      sessionID: "test-session",
+      agentToUse: "sisyphus-junior",
+      args: {
+        description: "test task",
+        prompt: "test prompt",
+        category: "quick",
+        run_in_background: false,
+        load_skills: [],
+      },
+      systemContent: undefined,
+      categoryModel: {
+        providerID: "openai",
+        modelID: "gpt-5.4",
+        variant: "medium",
+      },
+      toastManager: null,
+      taskId: undefined,
+    }
+
+    //#when
+    await sendSyncPrompt(mockClient, input)
+
+    //#then
+    bunExpect(promptAsync).toHaveBeenCalled()
+    bunExpect(promptArgs.body.agent).toBe("sisyphus-junior")
+    bunExpect(promptArgs.body.model).toEqual({
+      providerID: "openai",
+      modelID: "gpt-5.4",
+    })
+    bunExpect(promptArgs.body.variant).toBe("medium")
+  })
+
   bunTest("retries with promptSync for oracle when promptAsync fails with unexpected EOF", async () => {
     //#given
     const { sendSyncPrompt } = require("./sync-prompt-sender")

src/tools/delegate-task/sync-prompt-sender.ts

@@ -56,9 +56,7 @@ export async function sendSyncPrompt(
   const promptArgs = {
     path: { id: input.sessionID },
     body: {
-      // When a custom model is configured, omit the agent name so opencode's
-      // built-in agent fallback chain does not override the user-specified model.
-      ...(input.categoryModel ? {} : { agent: input.agentToUse }),
+      agent: input.agentToUse,
       system: input.systemContent,
       tools,
       parts: [createInternalAgentTextPart(effectivePrompt)],