[FEATURE]: /copy N to copy the Nth-latest assistant response #34044

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Today /copy copies the full session transcript. I'd like an argument form, /copy N, that copies just one assistant response: /copy 1 = latest, /copy 2 = 2nd-latest, and so on. Mirrors Claude Code's /copy / /copy N.

Why: often I only want to grab the last (or a specific recent) reply — a code block or answer — without copying the whole transcript and trimming it down.

Behavior:

• /copy (no arg) — unchanged, still copies the full transcript.
• /copy N — copies the Nth-latest assistant text response to the clipboard, with a success toast and an error toast if N is out of range.

Note this is a TUI/UI change, so flagging it for design review before I open a PR. Happy to implement if the approach sounds good.