ci: fix changeset workflow to support skip-changeset label

[RetricSu]

Summary

This PR modernizes the changeset CI workflow to support label-based bypassing, following best practices from popular open-source projects like Apollo GraphQL and Chakra UI.

Changes

• Added job-level conditionals to skip changeset check when `skip-changeset` or `dependencies` labels are present
• Added `labeled` trigger type so the workflow re-runs when labels are added/removed
• Added PR comment functionality using `actions/github-script@v7` for helpful contributor feedback
• Added companion `skipped` job to clearly show when the check is bypassed

How to Use

For contributors:

• If your PR needs a version bump: Add a changeset with `pnpm changeset`
• If your PR doesn't need a version bump (docs, CI, refactoring): Add the `skip-changeset` label
• For dependency updates: Add the `dependencies` label

Testing

1. Create a test PR
2. Verify the changeset check runs normally
3. Add `skip-changeset` label
4. Verify the "Changeset Check (Skipped)" status appears

Related

Work plan: `.sisyphus/plans/fix-changeset-ci.md`