chore: cleanup agent model references and defaults (#547)

* refactor(agents): remove unused model references

Consistent cleanup of agent model references across all agent files.

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

* fix(agents): use glm-4.7-free as default librarian model

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

* make playwright skill to be called more

src/agents/explore.ts

@@ -28,7 +28,6 @@ export function createExploreAgent(model: string = DEFAULT_MODEL): AgentConfig {
   const restrictions = createAgentToolRestrictions([
     "write",
     "edit",
-    "background_task",
   ])
 
   return {