fix aggregate changelog changeset commit lookup

[bdbch]

Changes Overview

Use Changesets' git helper to resolve the commit for each changeset when generating the aggregated changelog.

Implementation Approach

Replaced the custom per-file git log lookup with @changesets/git's getCommitsThatAddFiles() and kept the legacy changes.json fallback for older changesets.

Testing Done

Ran node --check scripts/aggregate-changeset.js.

Verification Steps

1. Run the aggregate changelog script for a release plan that includes one or more changesets.
2. Confirm the generated aggregated changelog links each entry to the commit or PR associated with that changeset rather than the repository's latest commit.

Additional Notes

This keeps commit resolution aligned with Changesets' own git utilities, including shallow clone handling.

Checklist

• I have created a changeset for this PR if necessary.
• My changes do not break the library.
• I have added tests where applicable.
• I have followed the project guidelines.
• I have fixed any lint issues.

Related Issues

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 52f8711

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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