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/features/background-agent/spawner.ts

@@ -135,9 +135,7 @@ export async function startTask(
   promptWithModelSuggestionRetry(client, {
     path: { id: sessionID },
     body: {
-      // 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 }),
+      agent: input.agent,
       ...(launchModel ? { model: launchModel } : {}),
       ...(launchVariant ? { variant: launchVariant } : {}),
       system: input.skillContent,
@@ -222,9 +220,7 @@ export async function resumeTask(
   client.session.promptAsync({
     path: { id: task.sessionID },
     body: {
-      // 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: task.agent }),
+      agent: task.agent,
       ...(resumeModel ? { model: resumeModel } : {}),
       ...(resumeVariant ? { variant: resumeVariant } : {}),
       tools: {