User-Agent header updated for HTTP requests

Patch Changes

• fae0396: Send a distinctive User-Agent header (releases-cli/<version> (+https://releases.sh)) on every outbound HTTP request — registry API calls, releases check feed probes, update checks, telemetry. Replaces the previous fall-through to Bun/undici's default node UA so api.releases.sh analytics and third-party site operators can identify CLI traffic.