Add Stacked PR Breadcrumbs

Generate a stacked PR breadcrumbs block to add at the beginning of a PR body.

Instructions

Use git branch-tree to figure out the branch hierarchy. Then lookup corresponding PR numbers for each branch and generate the breadcrumbs in this exact format:

> [!IMPORTANT]
> This is a stacked PR:
> - #<base_pr>
>     - #<current_pr> :point_left:
>         - #<child_pr>

Use gh to add the generated breadcrumbs block to the PR. If the PR already has a breadcrumbs block, replace it with the new one. Use a temp file for the PR body to ensure correct format.

Rules: