Clarify rationale for direct-child retargeting

[Phlogistique]

Motivation

• The previous README wording about retargeting stacked PRs was jargon-heavy and could be confusing about which PRs get their base updated after a squash merge.
• The intent is to convey that only the direct child of the landed PR is retargeted to trunk while indirect descendants remain based on their parent.

Description

• Reworded the explanation in README.md for step 5 to state plainly that direct child PRs are rebased onto trunk after the bottom change lands and higher PRs keep their parent.
• This is a documentation-only change and does not modify any script logic such as update-pr-stack.sh or gh pr edit --base usage.

Testing

• No automated tests were run because this change only updates documentation.
• The repository was updated and the change was committed successfully to the branch.

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Codex Task