feat: Add `--variant` flag to root opencode command

[ekweible]

Closes #7354

Summary

Adds --variant flag to root opencode command, matching existing support in opencode run.

Changes

• thread.ts: Add --variant option to yargs builder, pass to tui args
• args.tsx: Add variant to Args interface
• local.tsx: Initialize variant from CLI arg using model.variant.set()

---

This is my first contribution here, so please let me know if I've missed something! I didn't see any existing test coverage for the CLI args or the TUI local context, but I'm happy to add tests if desired.