Replace changeset reminder with a check

[andrewiggins]

What changed

• replace the commenting changeset reminder with a normal failing GitHub Actions check
• run the check again when pull request labels are added or removed, with 🤖 Skip Changeset as an explicit successful bypass
• detect release-relevant changes in non-private package source and metadata while ignoring tests, Markdown, examples, and private tooling
• remove pull request write access and document the new label for contributors

Validation

• pnpm run lint
• workflow YAML parsing
• node --check .github/scripts/changeset-relevant-files.mjs
• git diff --check
• local scenarios covering public source and package metadata, missing and present changesets, tests, docs, examples, Markdown, and private packages

No changeset is included because this only changes repository automation and documentation.