Auth token is memoized between CLI operations

The CLI’s shared macOS credential store now memoizes the resolved auth token in ServerAuthenticationController between calls (bounded by the token’s own expiry, single-flight, process-wide). The Xcode compilation cache (CAS) daemon and every other auth caller no longer perform a keychain read on every operation, removing the dominant per-op latency on hosted macOS runners where the keychain read serialized under concurrency.