[BUG] /terminal-setup is hidden in unsupported terminals; instead claude tries to parse the prompt

Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: 0.2.100
• Operating System: macOS 14.3
• Terminal: Ghostty

Bug Description

I saw a report that /terminal-setup did awesome stuff to set up claude code with the user's terminal. When I tried it, claude went off to research "terminal-setup" because I'm using Ghostty, which isn't supported by terminal-setup

It's weird and surprising that / commands are hidden when unsupported and don't offer up an error message.

There's an argument that unknown or unsupported /commands shouldn't be handed to the model, but should instead throw an error.

Steps to Reproduce

1. Use an unsupported terminal
2. /terminal-setup

Expected Behavior

An error message that /terminal-setup isn't supported on this platform.

Actual Behavior

Claude starts research

<img width="701" alt="Image" src="https://github.com/user-attachments/assets/27824def-ec85-4b4f-acde-cd817adfc23a" />

Additional Context

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