fix: remove stale PR registry files and untrack go.work.sum

Remove pr-registry.json and pr-registry-check.json which contain
outdated version 0.8.1-pr35 from a PR merged in Dec 2025. The current
release is v0.14.0. These files are generated by the PR build workflow
and uploaded to GitHub releases - they should not persist on the main
branch.

Also untrack go.work.sum which is already in .gitignore but was
committed before the gitignore entry was added.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jongio