Allow us to configure which branch new threads are automatically pulled from

[xMrAfonso]

As the title says, currently we can choose the default model for a project, if it should use the current thread or create a new worktree, if it should automatically pull etc. but we are not allowed to choose which branch it should pull from or select automatically. It currently selects origin/main which for someone who is working constantly in the staging branch is quite annoying to have to constantly change where it's pulling from every new thread.

[juliusmarminge]

Duplicate of #6866 — same request to configure the default base branch for new worktree threads. Please add any +1 / extra notes there.

[xMrAfonso]

OH that did not show up when I searched for it, must have searched it badly. Thank you!