Agent Skills: auth-signers

Signer abstraction patterns in Amethyst. Use when working with event signing, choosing between a local keypair (`NostrSignerInternal`), a remote NIP-46 bunker signer (`NostrSignerRemote`), or a NIP-55 Android external-app signer (`NostrSignerExternal`). Covers the abstract `NostrSigner` base class, `SignerResult` contract, how to wire a new flow that needs to sign events, and the security/UX trade-offs between signer kinds.