Allow /copy to target a specific assistant response by id

What problem are you trying to solve?

In the Codex CLI TUI, /copy currently copies the latest assistant response as markdown. That is useful, but it is hard to copy an earlier assistant response cleanly.

The current workaround is either:

• ask Codex to repeat the earlier answer and then run /copy, which burns tokens and adds friction;
• manually select text in the terminal, which often captures extra indentation, soft-wrap artifacts, or surrounding TUI formatting.

This is especially painful when copying markdown/code blocks from previous answers into chats, tickets, docs, or issue comments.

Related broader request: #2880. This issue is a narrower request for addressable copy of a specific message.

Proposed solution

Add a way for /copy to target a specific assistant response.

Possible command shapes:

/copy          # current behavior: copy latest assistant response
/copy 12       # copy assistant response with id 12
/copy a12      # copy assistant response with explicit assistant-message id
/copy last-2   # copy the previous assistant response

The TUI could expose compact ids for assistant messages, for example:

assistant #42
...

Alternatively, ids could stay out of the main transcript and be discoverable through /history or message metadata.

Why this would help

This would make /copy useful for more than the latest response while preserving clean markdown output and avoiding terminal-selection artifacts.