Improve readability of lowest unmerged pull request condition in stacked PRs CI doc

[ittakato]

Why:

The "Lowest unmerged pull request" bullet in Optimizing CI for stacked pull requests was hard to read: it fronted the reason ("Because it targets the stack base directly, ...") before stating the identifying condition, unlike the sibling "Top pull request" bullet.

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Restructures the bullet to follow the same sentence pattern as the "Top pull request" bullet in the same list:

Lowest unmerged pull request — the pull request currently at the bottom of the remaining stack, targeting the stack base directly. It is the pull request where github.event.pull_request.stack.base.ref equals github.event.pull_request.base.ref.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

🤖 Generated with Claude Code

[github-actions]

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Source	Review	Production	What Changed
pull-requests/how-tos/merge-and-close-pull-requests/optimizing-ci-for-stacked-pull-requests.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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