[FEATURE]: "Copy message" modal to copy any assistant message from conversation history #30548

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Describe the enhancement you want to request

ctrl+x y (messages_copy) only copies the last assistant message. There's no way to copy an older assistant message from the conversation.

I'd like a "Copy message" modal (similar to the fork/session timeline modal) that:

1. Lists all assistant messages in the current session (showing a truncated preview of each)
2. Allows navigating with up/down arrows and selecting one to copy to clipboard
3. Could be bound to a keybind (e.g. ctrl+x shift+y) or added as a slash command (/copy)

Current workaround:

Load the session transcript into text editor (either via session_export or session_copy), and copy the relevant content from there.

Related

This is a simpler subset of #22871 (selective message copying with multi-select). This issue focuses on a single-message picker as a first step toward the fuller feature. PR #15173 addresses #22871 with a more comprehensive implementation but has been stale for months.