Fix release tag pointing to wrong commit when master advances during build

[Copilot]

gh release create without --target defaults to the repository's default branch HEAD. Since the three-platform build matrix can take several minutes, any commit pushed to master during that window causes the release tag to land on a different commit than the one actually built and uploaded.

Change

• Add --target "${{ github.event.pull_request.merge_commit_sha }}" to gh release create in release.yml

gh release create "$TAG" \
  --title "Release $VERSION" \
  --generate-notes \
  --target "${{ github.event.pull_request.merge_commit_sha }}" \
  "${files[@]}"

All build jobs already checkout merge_commit_sha explicitly; this aligns the tag to that same commit so the tag, source, and artifacts are consistent.

Original prompt

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This section details on the original issue you should resolve

<issue_title>release tag の位置と artifact の不一致</issue_title>
<issue_description>release branchでworkflowが実行されるけど、ビルド中にmasterが更新されると、tagは最新のmasterについてしまっている。一方 release branch でタグを打つと、squash mergeしたときにmaster上にタグがいないことになってしまう。トリガーを見直す必要がありそう？</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes release tag の位置と artifact の不一致 #127

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