ci: add miniflare Workers integration tests for Cloudflare KV

## Goal
Complete the runtime coverage gap left when #11 closed: validate `edgeshield/storage/cloudflare-kv` against a real Workers-like environment.

## Scope
- Add miniflare (or wrangler dev) job in CI
- Smoke test KV `get` / `set` / `increment` / TTL through the adapter
- Run rate-limit integration test against Cloudflare KV binding mock

## Acceptance criteria
- [ ] CI job runs on PRs and main
- [ ] Adapter conformance suite passes under miniflare
- [ ] Document local dev command in README

## Related
- Closed #11 (Bun + Deno landed in v0.4.0)

Start at edgeshield/storage/cloudflare-kv, the existing adapter conformance suite, and the rate-limit integration test; then inspect the current CI configuration and README. Done means a Workers-like KV integration job runs on PRs and main, the get/set/increment/TTL and rate-limit checks pass, and the local development command is documented.

Written by the indexing model from the issue text.