/copy: allow selecting which part of the response to copy (e.g., just a code block)

### Summary

Today `/copy` copies the entire last assistant response to the clipboard. Often I only want a specific part — most commonly a single fenced code block from a longer answer that also includes prose, multiple commands, or several code snippets.

### Request

When I run `/copy`, present a picker of copyable segments from the last response, for example:

- The full response (current behavior, as default)
- Each fenced code block, listed individually with a short preview and language tag

Selecting one copies just that segment to the clipboard.

### Prior art

Claude Code does this: invoking its copy action on a response that contains fenced code shows a small menu so you can pick the whole response or any individual code block.

### Why

Right now if the response has prose plus a code block, I have to either:
- `/copy` the whole thing and then manually strip the prose in another editor, or
- Select with the mouse, which is fiddly across wrapped lines / terminal renderers and often picks up stray characters (see related copy-selection issues).

A built-in picker keyed off the markdown structure the CLI already renders would be much nicer.

首先追蹤現有的 /copy 動作以及 CLI 已經呈現的 markdown 結構，接著檢查觸發複製到剪貼簿的方式。完成條件是：/copy 預設提供完整回應，以及個別預覽、帶有語言標籤的 fenced 程式碼區塊，且選取任何選項時只複製該片段。

由索引模型根據 Issue 內容生成。