secretStorageKeys was untyped JS, passed to createClient behind `as any`,
which hid that getSecretStorageKey returned undefined where the SDK type
says null. Now TypeScript against the SDK's CryptoCallbacks, returns null,
and storePrivateKey keeps its own copy of the key. Unit test added.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5