Use `miniflare` for testing repo and recommend to users

Editor assessment

The subject states both the adoption and its recommendation — `miniflare` is used for repo testing and suggested to users — so the header communicates the tooling change and its outward advice without the diff, and chore fits the testing-setup work. Hygiene is clean: imperative mood, no trailing period, sensible length, no ticket junk. No body is needed for a contained tooling swap, and no footer is required. A line on what miniflare emulates, the Cloudflare runtime presumably, would add value; the message is already clear, accurate, and publishable.