Add support for sk-ssh-ed25519@openssh.com key type, enabling authentication with FIDO2/U2F hardware security keys (passkeys). SKP256 (sk-ecdsa) not included to keep scope minimal; can be added later

Changes

thrussh-keys:

• PublicKey::SKEd25519 variant with application field
• Signature::SKEd25519 variant with flags + counter authenticator data
• SK public key parsing/serialization (wire format and base64)
• SK signature verification: SHA256(application) || flags || counter || SHA256(message)
• SSH agent client support: identity listing, key blob serialization, sign request/response handling
• OpenSSH SK private key parsing: extract key_handle and application
• KeyPair::SKEd25519 variant for local FIDO2 signing via hardware token
• Local signing via ctap-hid-fido2::FidoKeyHid::get_assertion

thrussh:

• sk-ssh-ed25519@openssh.com added to algorithm negotiation lists
• Named and PubKey trait impls for SK keys
• Server-side auth: extract flags + counter from SK signature blob before verification