[FEATURE] Allow /btw (side chat) while an AskUserQuestion picker is displayed, so options can be clarified without dismissing it

Summary

When Claude Code displays a multiple-choice question (AskUserQuestion picker), the input line is taken over by the option picker and /btw cannot be invoked. If I do not understand what one of the options actually means, there is no way to ask about it in place. I have to dismiss the picker first, ask my question, and then get Claude to present the same choices again.

Please make /btw (the side chat) usable while a question picker is on screen, with the pending question preserved.

Problem

The picker is exactly the moment when I am most likely to need a clarification:

• Option labels are short (1–5 words) and the descriptions are one line. For anything non-trivial — a refactoring strategy, a library choice, a schema decision — that is often not enough to decide.
• /btw exists precisely for "ask a side question without disturbing the main thread", but it is unavailable at the one moment I need it most.
• Escaping out of the picker to ask costs a round trip and tells the model I declined, which can make it change course rather than simply re-ask.

Current workaround

1. Exit the selection screen.
2. Ask what the option means.
3. Ask Claude to present the choices again (or re-answer in free text and hope the intent carries).

Expected behavior

• /btw can be opened while an AskUserQuestion picker is pending.
• The pending question survives the side-chat interaction and is re-presented (still pending, same options) when the side chat is closed.
• No "user rejected / interrupted the tool use" signal is sent to the main thread as a side effect of using the side chat.

Ideally the side chat would have access to the pending question and its options as context, so "what does option 2 actually mean?" can be answered directly.

Steps to reproduce

1. Get Claude to present a multiple-choice question in the terminal UI (AskUserQuestion).
2. While the options are displayed, try to type /btw to ask what one of the options means.
3. The input is consumed by the option picker; /btw is not available.

Related issues

This is adjacent to, but not the same as, the following open issues. Those are about side-chat interaction destroying a pending prompt; this one is about the side chat not being reachable at all from the picker, and about being able to clarify an option before answering it. Happy to have it folded into one of them if maintainers prefer.

• #74959 — Interacting with the 'btw' section auto-rejects a pending AskUserQuestion
• #64568 — Pressing Esc to exit /btw mode rejects the pending tool-use prompt
• #89644 — AskUserQuestion modal hides upstream context, forcing decisions on option-summaries alone

Environment

• Claude Code CLI 2.1.260
• Windows 11 Pro (10.0.26200), PowerShell