Add trail ready and trail retarget

trail ready [<trail>] marks one change ready for review (--undo returns it to draft) and trail retarget --base <branch> changes the branch it merges into. Both resolve the change like trail approve and send a single-field PATCH to the change. The cell reports approve as unavailable until a change is ready, and there was no CLI path to mark it.

On a repository without an approval gate the cell stores readiness as false even when asked for true, so trail ready checks the response and fails instead of claiming success.