AbortSignal.reason upgrade propagation

Trap: preserving only caught errors named AbortError looks sufficient because fetch commonly rejects that way, but AbortController.abort(reason) may reject with a primitive, object, ordinary Error, or a distinct generic abort. Fix: whenever the caller signal is aborted, throw signal.reason unchanged before normalization, retry, or transport wrapping. Apply this across GitHub probes/pagination and GHCR token, manifest, tags, authenticated blob, redirected blob, and JSON-body reads.