Reddit API requires a non-empty User-Agent header per its API rules.
Without it, the JSON endpoint returns 403 even when Reddit is reachable,
causing doctor to falsely report the channel as unavailable.
Changes:
- Add _reddit_reachable() helper that probes Reddit JSON API with UA
- doctor check() now distinguishes 'actually unreachable' from 'no proxy'
- Users on home IPs get 'ok' when Reddit is reachable; warn only on real failure
Fixes#168
Panniantong