Track work as labelled issues and PRs, and check it in CI #60

Work is tracked as labelled GitHub issues, with release epics and a pinned roadmap (#34). PRs carry their issue's labels and close it.

What has to be in the repository for that:

• issue forms (Bug, Enhancement, Epic) and a PR template
• the rules in CLAUDE.md and CONTRIBUTING.md
• a CI check that fails a PR with no type label, a bug with no severity/*, or no closing reference

The labels, milestones and epics themselves already exist on GitHub.

Follow-up once #54 merges: move CLAUDE.md to AGENTS.md, as TypeKit does, so agents other than Claude read the same rules.