Selection action: one-click explain into side chat, plus shortcut

Summary

Selecting text in an assistant response currently offers Reply and Side chat, and both need a typed prompt. Add a one-click action on the selection, and a keyboard shortcut for it, that opens the side chat with the selected passage and an automatic explain prompt, sent without pressing Send.

Motivation

While reading a long response I often want a quick explanation of one term, error or decision. Today that means selecting the text, choosing Side chat, and typing a question. The typing is enough friction that I often skip the question.

A one-click "explain this" on a selection gives a quick understanding without writing a prompt, and the main conversation stays untouched.

Proposed behavior

1. A third action, e.g. Explain, beside Reply and Side chat in the selection popup.
2. It opens the side chat with the selection quoted and immediately sends an explain prompt (e.g. "Explain this in more detail").
3. A keybinding (user-rebindable) does the same with the current selection, and sends into the already-open side chat if there is one.
4. The side chat's answer does not enter the main conversation's context.
5. The prompt text is configurable.

Why plugins can't do this

I checked the plugin/mod API declarations (Claude Code 2.1.286): there is no event or value for the current text selection, no call to read the clipboard (only to write it), and no call to open or send into the side chat. $.prompt.submit sends into the main conversation only. This can't be built as a mod today.