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/manager.ts

@@ -515,9 +515,7 @@ export class BackgroundManager {
     promptWithModelSuggestionRetry(this.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,
@@ -794,9 +792,7 @@ export class BackgroundManager {
     this.client.session.promptAsync({
       path: { id: existingTask.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: existingTask.agent }),
+        agent: existingTask.agent,
         ...(resumeModel ? { model: resumeModel } : {}),
         ...(resumeVariant ? { variant: resumeVariant } : {}),
         tools: (() => {