Check changesets exist for changed packages

[josephjclark]

Short Description

Fixes #1476

QA Notes

Tests I'm running in commits to this repo:

• Action does not run if label is not added
• Action does not fail if there are no changes
• Action fails if there is a change but not changeset
• Action does not fail if there are multiple changes with changesets
• Action fails if there are existing changesets, and a new package is changed

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

You can read more details in our
Responsible AI Policy

Release branch checklist

Delete this section if this is not a release PR.

If this IS a release branch:

• Run pnpm changeset version from root to bump versions
• Run pnpm install
• Commit the new version numbers
• Run pnpm changeset tag to generate tags
• Push tags git push --tags

Tags may need updating if commits come in after the tags are first generated.