[Feature]: Model inherit from Sisyphus

Prerequisites

• I will write this issue in English (see our Language Policy)
• I have searched existing issues and discussions to avoid duplicates
• This feature request is specific to oh-my-opencode (not OpenCode core)
• I have read the documentation or asked an AI coding agent with this project's GitHub URL loaded and couldn't find the answer

Problem Description

Currently, to use the same model across all agents, users must manually configure each agent to point to the same category or model. This is verbose and requires updating every agent individually when the model changes.

Proposed Solution

Add a config option to make all agents automatically inherit Sisyphus's model. For example:

{
  "agents": {
    "inherit_from_sisyphus": true
  }
}
Or a more flexible approach:
{
  "agents": {
    "default_inherit": "sisyphus"
  }

### Alternatives Considered

Using a shared category and pointing each agent to it manually — but this requires listing every agent and doesn't automatically update when Sisyphus changes

### Doctor Output (Optional)

```shell

Additional Context

This would simplify configuration for users who want consistency across all agents without verbose per-agent overrides.

Feature Type

Configuration Option

Contribution

• I'm willing to submit a PR for this feature
• I can help with testing
• I can help with documentation