fix(command): validate model before executing slash command

[devxoul]

Background

Slash commands with invalid model fails silently. It will be great if we can display the error message.

For example:

~/.config/opencode/command/hello.md

---
description: Greet the user warmly
model: anthropic/claude-haiku
---

You are a friendly assistant who greets the user warmly. Greet the user warmly and cheerfully.

Summary

• Validate model existence before executing slash commands
• Show friendly error message with fuzzy suggestions when model not found

Preview

[rekram1-node]

/review

[github-actions[bot]]

lgtm