feat(passkey): add WebAuthn passkey ceremony package

[jmgilman]

Summary

• add a new passkey package wrapping github.com/go-webauthn/webauthn ceremonies
• keep passkeys as proof material that returns authkit.Identity for onboarding/exchange
• define passkey-local store/user/credential contracts with fake-backed package tests only
• require user verification, enforce ceremony timeouts, handle clone warnings fail-closed, and validate registration/storage binding invariants

Validation

• go test -count=1 ./passkey
• go test -count=1 ./...
• moon run root:check --summary minimal
• git diff --check