Vim mode: support system clipboard registers ("*, "+) or clipboard=unnamed for cross-prompt yank/paste

Problem

In vim mode, yy yanks to an internal register that's destroyed when the prompt is submitted. There's no way to preserve the yanked text for p at the next prompt. This breaks the most common vim composition pattern (yank fragment from one input, paste into another).

What's missing

The vim mode currently supports only the unnamed register. None of the standard vim register escape hatches exist:

• No system clipboard registers ("*, "+)
• No named registers ("a–"z)
• No clipboard=unnamed / clipboard=unnamedplus setting equivalent
• No :registers introspection
• No cross-prompt persistence of even the unnamed register

So there's no way — within vim mode — to make yy then p (across a submission boundary) work.

Use case

Composing a prompt, realizing the wording is better suited for the next message, yanking it, submitting, then p at the new prompt. Standard vim muscle memory; currently impossible.

Suggested fix (any of these would solve it)

1. clipboard=unnamed parity — a settings.json key like "vimClipboardUnnamed": true that routes y/yy to the system clipboard automatically. Lowest-friction option.
2. Explicit system clipboard registers — recognize "* and "+ so users can opt in per-yank with "*yy and paste with "*p.
3. Persistent unnamed register across prompts — keep the last-yanked text in the unnamed register across submissions within a session.

Related

• #5005 requests a general clipboard-copy shortcut for the current prompt — overlapping but distinct (this is about register semantics inside vim mode, not a one-shot copy keystroke).

Thanks — Claude Code's vim mode is otherwise excellent.