Allow configuring default model for built-in agents

Feature Request

Allow users to configure the default model for built-in agents (Explore, Plan, etc.) through settings.

Use Case

The built-in Explore agent defaults to Haiku for speed, but some users prefer Sonnet for better quality results when exploring codebases. Currently, the only way to use a different model is to specify it on every Task tool call.

Proposed Solution

Add an agentDefaults section to settings.json:

{
  "agentDefaults": {
    "Explore": {
      "model": "sonnet"
    },
    "Plan": {
      "model": "sonnet"
    }
  }
}

This would let users override the default model for any built-in agent type while preserving the current defaults for those who don't configure it.

Alternatives Considered

• Creating custom agents via plugins (works but duplicates built-in functionality)
• Specifying model on every Task call (tedious and easy to forget)