[FEATURE]: Commands Model Variant Specification

[koichincom]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I'm willing to implement this, but since it involves some design decisions, I wanted to open an issue first.

Summary

Add a variant field to the commands feature. Currently, command definitions in command/*.md or opencode.json(c) only allow selecting the model or agent, but not the model variant.

Motivation

While variants can be applied globally or per-agent, it is useful to bake specific reasoning efforts into commands. For example, a command designed for complex planning or code review might always want to use a high thinking variant of a lightweight model, even if the user's global preference is set to minimal. This ensures specific tasks have the reasoning budget they need without requiring the user to manually switch variants before running the command.

Proposed Priority

If implemented, I suggest the following priority for variant resolution:

1. Explicit runtime selection (CLI --variant flag or TUI ctrl+t)
2. Command definition variant (The new field)
3. Agent default variant (as implemented in #7140)
4. None (Model default)