[FEATURE]: Add missing --variant option for standard "opencode" command to start new session.

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

For example, I'd like to:

alias oc-fast='OPENCODE_DISABLE_MODELS_FETCH=1 opencode --model openai/gpt-5.6-sol --variant low`

But currently, there is no way that I can find to specify the variant on the CLI for the normal start session mode – there is a --variant CLI flag already for the run command. This feature was suggested before but was auto-closed and not re-opened by the author: https://github.com/anomalyco/opencode/issues/7354 and he opened a PR here: https://github.com/anomalyco/opencode/pull/7358