[Feature Request] Built-in subagents should support configurable or inherited model selection

Question

**Bug Description**
 The built-in subagents (general-purpose, Plan, etc.) are hardcoded to use Sonnet,
  but now that Opus 4.5 is the flagship model, it feels like a downgrade when
  subagents spawn. Would be great if we could either configure the model for
  built-in agents, or have them default to "inherit" from the parent conversation.
  Creating custom agents to override built-ins works but feels hacky.

**Environment Info**
- Platform: darwin
- Terminal: WarpTerminal
- Version: 2.0.69
- Fee