Adds GitHub stacked pull request creation: a contextual "Stack this PR above #N" option that appears only when the selected base branch has an open PR, plus the main-process stack preflight and registration.
Also reworks the create-review composer for cohesion: shadcn Checkbox and Label primitives, base label above a full-width searchable combobox with attached results, keyboard navigation, and a unified field skin, spacing and typography scale.
Verified end to end against real GitHub: extending an existing stack and creating a new one.