Support changelog entries without package frontmatter (chore/CI-only changes)

[emmajam]

Problem

When a PR only touches CI, docs, or other non-package files (e.g. .github/workflows/), there's no relevant package to bump. Currently the changelog check fails with:

Error: failed to parse changelog ai-generated: missing frontmatter

This happens because the AI generates a changelog entry with empty frontmatter (---\n---) since no crate/package was actually changed.

Suggestion

Support a way to create changelog entries that don't bump any package, for example:

• Allow empty frontmatter (---\n---) to mean "no version bump, just a changelog note"
• Or support a special key like type: chore that skips version bumping
• Or allow the check to pass without a changelog entry when only non-package files are changed

Workaround

Currently we work around this by picking an arbitrary package and giving it a patch bump, which is misleading since the package didn't actually change.