Use GitHub's renamed page_data/stacks endpoint

[dejorrit]

GitHub renamed the route behind the stack badge popup from /pull/{n}/page_data/stack to /pull/{n}/page_data/stacks. The old path now returns 404 for every PR. Stacks were still grouped, but only because each one fell back to loading the full PR page and reading the embedded JSON. This PR switches to the new path, so it's one small JSON request per stack again.

Evidence

• GitHub's current frontend bundle builds the URL as /${owner}/pull/${n}/page_data/${routes.stack}, and the route table maps stack:"stacks".
• On checkout: resolve remote stack by branch name github/gh-stack#477, with the extension's headers:
  • page_data/stack returns 404;
  • page_data/stacks returns 200 with the same { stack: { id, number, size, baseBranch, pulls[...] } } shape. A PR that isn't stacked returns stack: null.
• The live response runs through core.normalizeStack unchanged: stack 590391, size 3, PRs 477/475/469.

Testing

• node --test passes 10/10.
• Checked anonymously on a public repo only. Reload the extension on a signed-in private repo list to confirm the stacks request returns 200 and the PR page requests are gone.

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