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

* refactor(agents): remove unused model references

Consistent cleanup of agent model references across all agent files.

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

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* make playwright skill to be called more

src/agents/oracle.ts

@@ -102,7 +102,6 @@ export function createOracleAgent(model: string = DEFAULT_MODEL): AgentConfig {
     "write",
     "edit",
     "task",
-    "background_task",
   ])
 
   const base = {