Feature request: add configuration option for PRs to be opened as drafts by default when using `submit` without `--auto`

Great tool, loving it!

I like to submit using `gh stack submit` without the `--auto` flag.

However, I still want all my PRs to open as drafts first (while I wait for CI to run, add images, etc.)

At the moment, this means I need to manually toggle each PR in the stack from "Ready" to "Draft" status after editing its description.

I would love a configuration option that set the default PR status to "Draft" rather than "Ready" when NOT using the `--auto` flag.

Start at the `gh stack submit` command and its handling of the `--auto` flag, then trace where PR status and configuration defaults are processed. Done means a configuration option makes PRs open as drafts when `submit` is used without `--auto`, with the existing behavior preserved otherwise.

索引モデルが issue の本文から書いたものです。