Subagents should default to the model picker current model

## Problem

Currently, every subagent (sisyphus, hephaestus, oracle, librarian, explore, multimodal-looker, prometheus, metis, momus, atlas, sisyphus-junior) requires an explicit `model` field to be set in `oh-my-openagent.json`. Without it, they fall back to whatever default the plugin picks, which does not respect the model the user has selected in the OpenCode Electron app model picker.

## Feature Request

Subagents should automatically inherit the model currently selected in the model picker. When a user switches the model in the picker (e.g., from Deepseek V4 Flash Free to Deepseek V4 Pro), all subagents should follow suit — unless the agent has an explicit `model` override in `oh-my-openagent.json`.

This would allow users to:
- Switch all agents models site-wide by changing the picker
- Still override individual agents when needed (e.g., Prometheus needs a stronger model)
- Avoid manually pinning 11 agents just to match what the picker already shows