[Feature]: Add `/copy` command

[kowyo]

Feature description

Add a /copy command to allow:

1. Copy the last assistant response to clipboard.
2. Pass a number N to copy the Nth-latest response: /copy 2 copies the second-to-last.
3. Press w in the picker to write the selection to a file instead of the clipboard, which is useful over SSH

Problem it solves

https://www.reddit.com/r/ClaudeAI/comments/1m8ayq0/how_can_i_copy_text_output_by_claude_code_in_the/

Proposed solution

TBD

Screenshots or mockups

No response

Additional context

No response