`gh stack init` should start up-to-date

`git town hack` always ensures that when you start a new branch, you are up-to-date on the source branch first - this is extremely useful. `gh stack init` should either have the same behavior by default, or offer a (defaultable) flag to do the same.

Start at the `gh stack init` command entry point and trace how it selects and prepares the source branch. Compare that flow with the up-to-date behavior described for `git town hack`, then run the existing CLI tests or command checks. Done means initialization reliably starts from an up-to-date source branch, with the chosen default or flag behavior covered.

索引モデルが issue の本文から書いたものです。