Point the stacked-PR link at the current GitHub docs page

The linked URL 404s; add the use case to the skill description so the
skill is found when a stack's bottom layer has merged.

Author: koppor

skills/magic-merge-commit/SKILL.md

@@ -3,6 +3,7 @@ name: magic-merge-commit
 description: Fix a Git merge conflict caused by a squash-merged base branch. Use
   when a PR can't merge `main` cleanly because its base branch was squash-merged,
   and Git sees the old commits as conflicting with the squashed commit in `main`.
+  Typical for GitHub stacked pull requests after the bottom layer merged.
   Creates a "magic" merge commit linking the branch to the squashed history.
 ---
 
@@ -22,7 +23,7 @@ All of the following are true:
 - That other PR was **squash-merged** into `main`.
 - Merging `main` into the current branch now conflicts.
 
-Typical case: a [GitHub stacked pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-stacked-pull-requests).
+Typical case: a [GitHub stacked pull request](https://docs.github.com/en/pull-requests/get-started/about-stacked-prs).
 The bottom layer gets squash-merged into `main`, and the layer above it — whose
 branch still carries the pre-squash commits — no longer merges `main` cleanly.
 Run this skill in the upper layer's worktree with the bottom PR's number.