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.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)],