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.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: (() => {