chore: clean Changesets output for root CHANGELOG

[techiejd]

Summary

PR #47 (the automatic Version Packages PR) produced an ugly root CHANGELOG.md diff: ## 0.7.2 was inserted between the # Changelog H1 and the intro paragraph, and had no bullet content because the changeset from #46 only named the adapter packages — the root was being silently pulled along by the fixed group with no narrative.

This PR fixes both issues so the next Version Packages PR is clean.

Changes

• .changeset/fix-adapter-type-declarations.md: add payloadcms-vectorize to the frontmatter. With the fixed group in .changeset/config.json, the root was already being bumped — but Changesets only writes a narrative bullet for packages explicitly named in the changeset. Naming the root fixes the empty-section problem.
• CHANGELOG.md: remove the intro paragraph between # Changelog and the first version section. The default @changesets/changelog-github formatter inserts new content right after the H1, so anything between the H1 and the first ## gets awkwardly sandwiched below the new release header.

Expected output after merge

I ran changeset:version locally to preview. The root CHANGELOG.md now gets a proper entry:

# Changelog

## 0.7.2

### Patch Changes

- [#46](https://github.com/techiejd/payloadcms-vectorize/pull/46) [\`664b2b6\`](...) Thanks [@stevenlafl](...)! - Fix missing TypeScript declarations in \`@payloadcms-vectorize/pg\` and \`@payloadcms-vectorize/cf\`. The build now runs \`tsc\` before SWC...

## 0.7.1 - 2026-03-20
...

Follow-up

PR #47 was closed and the changeset-release/main branch deleted so the release workflow regenerates a clean Version Packages PR when this one merges.

Test plan

• Merge this PR.
• Confirm release.yml fires, runs changeset:version, and opens a new "chore: version packages" PR.
• Review that new PR — root CHANGELOG.md should now have a full ## 0.7.2 section with PR link + @stevenlafl credit, not an empty header.
• Merge the Version Packages PR.
• Confirm release.yml fires again, changeset publish authenticates via Trusted Publishing OIDC, and all three packages publish at 0.7.2 on npm.