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:

1. •Each level of the stack should be indented with 4 spaces more than the previous level
2. •The current PR (the one being created/edited) should have :point_left: at the end
3. •Always start with the > [!IMPORTANT] callout
4. •Always include the header line > This is a stacked PR:
5. •Each PR number should be prefixed with #
6. •All lines must start with > to maintain the blockquote format

Example with 3 PRs in stack:

If the user provides: base PR #948, then #950, and current PR #951

Output:

> [!IMPORTANT]
> This is a stacked PR:
> - #948
>     - #950
>         - #951 :point_left:

Example with 2 PRs in stack:

If the user provides: base PR #948, current PR #950

Output:

> [!IMPORTANT]
> This is a stacked PR:
> - #948
>     - #950 :point_left: