gptel-transient: ensure point is after prompt

[LemonBreezes]

Basically, this code changes this:

to:

[karthink]

The reason I don't do this by default is because

1. I want the user to realize that the Rewrite: text is not required, i.e. not part of some pre-designed prompt they need to respect, so they can type anything, and
2. It's easier to C-k or C-e and start typing than having to C-0 k from the end. (Most users also don't know about C-0 k or how to kill to the beginning of a line, and don't use evil-mode in the minibuffer either.)