chore(ci): remove orphaned changeset for ignored package

The manifest-ui package was added to the changeset ignore list in PR #639,
but the existing changeset file (fix-sidebar-links.md) was not removed.
This caused the release workflow to fail with "No commits between main
and changeset-release/main" because changesets ignored the version bump
but still tried to create a PR.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: brunobuddy