[FEATURE] Show Opus Plan in /model command options

Problem

When running /model, the interactive picker shows individual models (Sonnet, Opus, Haiku) but doesn't include composite model modes like "Opus Plan" (opusplan). Users who don't know about opusplan can't discover it through the UI. You have to already know about it and type it manually.

Proposed solution

Add opusplan (and any other composite modes) as selectable options in the /model picker. Each entry should show what it does — e.g., "Opus Plan — Opus for planning, Sonnet for implementation".

<img width="727" height="190" alt="Image" src="https://github.com/user-attachments/assets/b513f061-af1d-42b4-84d4-bbb1f43b9ef8" />

Priority

Medium

Category

CLI commands and flags