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

### 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

Start by reproducing the documented steps with experimental features enabled, `/model auto`, and an explicit supported model. Read the rubber duck documentation and `/help` handling to identify where availability is determined. Done means the behavior under auto is either supported consistently or clearly explained in the docs or in-session.

索引モデルが issue の本文から書いたものです。