- GitHubExecutor implements Executor interface against api.github.com/search/code
- Retry-After honored once for 403/429; ctx cancel respected during sleep
- ErrMissingAuth wrapped for empty token AND 401 server response
- 8 httptest-backed subtests cover success/limit-cap/retry/rate-limit/401/422/source
- Zero new dependencies (stdlib net/http + net/url only)
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