/rubber-duck availability is unclear under /model auto; docs should clarify or command should work when auto resolves to an eligible model

[pwarnock]

What happened?

/rubber-duck was not available/discoverable in-session while using /model auto, despite:

• latest local Copilot CLI installed
• experimental enabled
• rubber duck documented as manually invokable
• auto typically resolving to a GPT/Claude family model

This makes it unclear whether:

1. /rubber-duck is intentionally unsupported under /model auto
2. it should work whenever auto resolves to an eligible model
3. command availability is determined at session start in a way that can leave the slash command hidden/unavailable

Why this is confusing

The docs say rubber duck is available when the main agent is using a Claude or GPT large model, but they do not clearly explain how /model auto affects /rubber-duck availability.

From a user perspective, if auto resolves to an eligible model, /rubber-duck should either:

• work, or
• clearly explain why it is unavailable

Reproduction

1. Start Copilot CLI with experimental enabled
2. Use /model auto
3. Try to invoke /rubber-duck ...
4. Observe that the command may not appear or may not be available in-session
5. Switch to an explicit supported model
6. Observe that /rubber-duck works or becomes available

Expected behavior

One of these should be true:

1. /rubber-duck works under /model auto whenever auto resolves to an eligible model, or
2. the docs explicitly state that /model auto may disable/hide /rubber-duck, or
3. the CLI surfaces a clear explanation in-session when /rubber-duck is unavailable

Suggested fix

• Preferably make /rubber-duck work when auto resolves to an eligible model
• Otherwise document the limitation clearly in the rubber duck docs and/or /help
• Ideally surface a direct in-product explanation rather than silently hiding the command