feat(tui): reinsert forked message text in prompt text input box when forking session (resolves #5495)

[ariane-emory]

This PR adjusts the behaviour when forking a session so that the text of the message on which the session is being forked is reinserted in the prompt text input box.

This improves ergonomics and can save two clicks and a keystroke by eliminating the need for the currently-common pattern of copying a message (two clicks) before forking on it so that you can begin the fork using a modified version of that message (one keystroke to past the copied message back into the prompt text input box).

Resolves #5495.

[rekram1-node]

/review

[ariane-emory]

The bot's change introduced an error :( Fixing presently.

[ariane-emory]

Fixed.

[ariane-emory]

@rekram1-node Should be ready to come in for a safe landing now, yeah?