[codex] Fix Changesets changelog generation

[RhysSullivan]

Summary

Fixes the Version Packages changelog output generated by Changesets.

The release PR body was pasting placeholder CHANGELOG.md prose because .changeset/config.json disabled changelog generation with changelog: false. When changeset version ran, package versions changed but package changelog sections were not generated, so changesets/action had no real release entries to include.

This PR:

• turns package changelog generation back on with @changesets/cli/changelog
• normalizes seed-only CHANGELOG.md files to H1-only files using package names
• updates the changelog seed checker so future seed files stay H1-only
• documents that .changeset/*.md bodies are the package changelog entries, while apps/cli/release-notes/next.md remains the GitHub Release body

Validation

• rehearsed bun run changeset:version locally and confirmed generated sections no longer include placeholder text
• bun run lint:changelog-stubs
• bun run format:check

References

• Changesets config supports changelog generators: https://github.com/changesets/changesets/blob/main/docs/config-file-options.md
• Changesets Action creates/updates the Version Packages PR from changeset version: https://github.com/changesets/action

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