…nding

Use refs for refreshPullRequestsAtom and registry to stabilize the
auto-refresh useEffect dependency array. Add lastAutoRefreshedKeyRef
to prevent duplicate auto-refreshes for the same cache key.

This stops the runaway Effect fiber creation that caused high CPU
and memory usage when the GitHub API returned errors.

- Remove redundant auto-refresh useEffect that double-fetched with
  pullRequestsAtom's own reactive recompute.
- Add refreshInFlightRef guard so rapid refresh calls (startup,
  focus return, idle refresh, user keypress) cannot overlap.
- Detect GitHub rate-limit errors in Effect.retry and skip retries,
  failing fast instead of spawning 6 retry processes each.
- Clear in-flight flag when pullRequestResult exits waiting state.